Morrowind ESP File Format

Overview

The esp is basically a list of objects. Each object represents something that has been added or changed from the ESM.

The first 362 bytes of the ESP are the header. (This is true for most ESP files, but can vary if the ESP uses a different master file than Morrowind.esm, or if it is dependent upon more than one master file). The header contains the author's name, a description of the mod, the master filename, etc.

Objects begin following the header. Each object starts with its type and length. Then come the subobjects ((type + length + data) + subobject + ...). The type is a 4 letter word (Ex: DOOR, LIGH, STAT). The length field of some larger objects is 12 bytes, but the length field of most objects and subobjects is only 4 bytes.

Note: It appears that the extra 8 bytes of "length data" may actually hold bitflag information. E.g., if an item is marked as being "blocked" or "references persist", the last 8 bytes of this "length" field change. We just haven't pinned down what it all means at this time.

The length, as with all binary values in the ESP file, is written in little endian form, base 256, so that the least significant byte comes first, and the most-significant comes last: A*256^0 + B*256^1 + C*256^2....

In addition, most "name" fields in Morrowind are limited to 31 characters in length. In the ESP file, these fields are often written as 32-byte strings, with extra space at the end filled by zeroes, and there must be at least one zero at the end, which forces the maximum name length to be 31.

Object Tags

Tag Description
ACTI Activator
ALCH Alchemy
APPA Apparatus
ARMO Armor
BOOK Book
BODY Bodypart
CELL Cell
CLAS NPC / Player Class (e.g., Paladin, Spellsword)
CLOT Clothing
CONT Container
CREA Creature
DOOR Door
DIAL Dialog
ENCH Enchanting
FACT Faction
FRMR Object Reference (rather than a whole new object or copy of an object)
GLOB Global variable
INFO Info/Response
INGR Ingredient
LAND Changes in landscape heightmap
LEVC Leveled creature
LEVI Leveled item
LIGH Light
LTEX Changes in landscape texture
MGEF Magic effect
MISC Misc item
NPC_ Npc
PGRD Path grid
PROB Probe
REPA Repair item
SCPT Script
SKIL Skill
SNDG Sound: defines what kind of sound a creature/thing makes.
SOUN Creates a new type of sound. So SNDG will refer to a SOUN, which refers to a WAV file.
SPEL Spell
STAT Static
WEAP Weapon
Field Names
BNAM NPC Facetype
CNAM Sound file used by a creature or NPC.
FLTV Additional information for a reference to an object. Used to hold lock information for chests and doors.
FNAM Friendly Name: name the user will see in-game.
ITEX Inventory pic for items
KNAM Name of an associated object. E.g., could be the name of the key that unlocks this chest or door.
MODL NPC Model
NAME Unique identifier
NAM0 Not sure. Appears before a list of object references in a cell.?
NPCO Inventory of a creature or NPC; contents of a container.
RNAM NPC Race
SCHD Script header (Name)
SCDT Script Data
SCRI Name of a script attached to an object.
SCTX Script text

Attribute IDs

Player attributes are assigned an index from 0 to 8. When an NPC, Class, or other object references an attribute, it will do so by this unique ID. Attributes map to values as follows.
  1. Strength
  2. Intelligence
  3. Willpower
  4. Agility
  5. Speed
  6. Endurance
  7. Personality
  8. Luck

Skill IDs

Player skills are assigned an index from 0 to 26. When an NPC, Class, or other object (such as a book that increases a skill) references a skill, it will do so by this unique ID. Skills map to values as follows. (We show the hex value of the skill in parentheses, for all values > 9).
  1. Block
  2. Armorer
  3. Medium Armor
  4. Heavy Armor
  5. Blunt
  6. Longblade
  7. Axe
  8. Spear
  9. Athletics
  10. Enchant
  11. Destruction (0x0A)
  12. Alteration (0x0B)
  13. Illusion (0x0C)
  14. Conjuration (0x0D)
  15. Mysticism (0x0E)
  16. Restoration (0x0F)
  17. Alchemy (0x10)
  18. Unarmored (0x11)
  19. Security (0x12)
  20. Sneak (0x13)
  21. Acrobatics (0x14)
  22. Light Armor (0x15)
  23. Shortblade (0x16)
  24. Marksman (0x17)
  25. Mercantile (0x18)
  26. Speechcraft (0x19)
  27. Hand to Hand (0x1A)

Ordering

The TES editor tends to put things in a certain order into the ESP file, but order does not appear to be all that significant. We have seen ESP files with different orders load fine into the editor. However, the editor may be more forgiving about this than the game. With home-grown ESP files, it is typically a good idea to load the file into the editor, make some minor modification, and save it, to ensure the object order is correct.

Individual sections are left out of the ESP file if nothing of that type has been added or changed.

Details

If you have more information about missing fields, the meaning of "unknown" fields, or if you know that information presented here is wrong, please let us know. We'll give credit to people who submit input to this probject.

The original version of the editor generated only what you told it to. Later versions, especially if you have official expansion packs installed, tend to add a lot of extra content, especially dialog. Not sure why that is, but it is annoying.

Field Type Length Description

Header

 Warning. Be careful about assuming a fixed length for the header. Because the header contains the master filename, the length of the header can vary....
File Signature String 4 This is always "TES3" to indicate that it's an elder scrolls 3 file.
Header Size Integer 4 Indicates how big the header is. Usually is 0x5A 0x01 0x00 0x00.
Reserved Byte 8 These bytes should all be NULL.
Header String 4 "HEDR"
Header Size Binary 4 Distance from the end of the Header Size to the start of the MAST entry. Usually this is 0x2C 0x01 0x00 0x00.
Version Number Float 4 Version number (1.2) of the file, as a floating point number:
0x9A 0x99 0x99 0x3F
Unknown Binary 4 Four null values, unknown purpose: 0x00 0x00 0x00 0x00
Author Name String 32 Unused characters should be NULL.
Description String 260 User's description of this mod. Unused characters should be NULL.
Master File String 4 "MAST"
Master Filename Length Binary 4 Indicates the length, in bytes, of the master filename string that follows, including the required trailing NULL. If you're using "Morrowind.esm" as the master file (probably), then this value is 0x0E.
Master Filename String Varies This is a null-terminated string, typically "Morrowind.esm" but it can be different if you're using a custom ESM file.
Data String 4 "DATA"
Data Length Integer 4 0x08 0x00 0x00 0x00
Data Integer 8 Size, in bytes, of the master file:
0x1C 0xF9 0xC1 0x04 0x00 0x00 0x00 0x00

Script Data
Script String 4 "SCPT"
Script Length Integer 12 Length of the script, from the "SCHD" tag to the end of the script.
Script Header String 4 "SCHD"
SCHD Length Integer 4 Appears to always be 0x34.
SCHD Data String Varies This field contains the name of the script, but also contains additional non-null data at odd offsets, so this is probably also a compiled portion of the script.
Script Variables (Compiled) String 4 "SCVR"
SCVR Length Integer 4 Length of the variable section.
SCVR Data String Varies Each variable name is listed, separated by a null.
Compiled Script String 4 "SCDT"
SCDT Length Integer 4 Length of the SCDT field
SCDT Data Binary Varies Compiled version of the script.
Script Text String 4 "SCTX"
SCTX Length Integer 4 Length of the SCTX field
Script Text Data String Varies Uncompiled, text version of the script.

Class Data

 This is rare, but if you decide to create a new type of NPC or player class, this is what you would use.
Class String 4 "CLAS"
Class Size Integer 12 Length of the Class record.
Name String 4 "NAME"
Name Length Integer 4 Length of the unique identifier for this class.
Name Data String Varies Unique ID name of the class. Ex., "Fighter".
Friendly Name String 4 "FNAM"
FNAM Length Integer 4 Length of the friendly name, including trailing null.
FNAM Data String Varies User-friendly name for this class. Ex., "Fighter".
Class Data String 4 "CLDT"
CLDT Length Integer 4 Typically is 0x3C: 15 4-byte values.
CLDT Data Set of Integers 0x3C Note: These values are IDs that map either to Attributes or Skills.

Note 2: The Skills are listed with alternating Major and Minor skills, Minor skills starting first. So the order is Minor, Major, Minor, Major.... One thing that is confusing is that the Major skills come in the same order as they appear in the Class dialog box in the TES editor. The Minor skills, though, are in a different order. As far as I can tell, the order of Minor skills in the ESP maps to the dialog as: 1,4,5,3,2. The order only matters if you are trying to map values you see in the ESP to values you see in the dialog box.

  1. Primary Attribute #1
  2. Primary Attribute #2
  3. Specialization: 0 = Combat; 1 = Magic; 2= Stealth
  4. Minor Skill #1
  5. Major Skill #1
  6. Minor Skill #2
  7. Major Skill #2
  8. Minor Skill #3
  9. Major Skill #3
  10. Minor Skill #4
  11. Major Skill #4
  12. Minor Skill #5
  13. Major Skill #5
  14. If the class is playable, value is (01 00 00 00); otherwise is (00 00 00 00)
  15. A set of flags for the "Auto-calc Buy / Sell" flags. Not entirely sure what this means exactly, but the bitmasks are as follows:
    • Weapons: 0x0001
    • Armor: 0x0002
    • Clothing: 0x0004
    • Books: 0x0008
    • Ingredients: 0x0010
    • Lockpicks: 0x0020
    • Probes: 0x0040
    • Lights: 0x0080
    • Apparatus: 0x0100
    • Repair Items: 0x0200
    • Miscellaneous: 0x0400
    • Spells: 0x0800
    • Magic Items: 0x1000
    • Potions: 0x2000
    • Training: 0x4000
    • Spellmaking: 0x8000
    • Enchanting: 0x10000
    • Repair: 0x20000
Class Description String 4 "DESC"
Desc Length Integer 4 Length of the description, including trailing NULL.
Desc Data String Varies Describes the class.

Faction Data

 This is rare, but if you decide to create a new player or NPC faction, this is what you would use. (Ex., Mages Guild, Redoran, etc.)
Faction String 4 "FACT"
Faction Size Integer 12 Length of the Faction record.
Name String 4 "NAME"
Name Length Integer 4 Length of the unique identifier for this faction.
Name Data String Varies Unique name of the faction. Ex., "Axe Swingers".
Friendly Name String 4 "FNAM"
FNAM Length Integer 4 Length of the friendly name, including trailing null.
FNAM Data String Varies User-friendly name for this faction. Ex., "Swinging Axes".
Rank Names String 4 "RNAM"
RNAM Length Integer 4 Generally always 0x20, with extra space padded by nulls.
RNAM Data String 0x20 Rank name such as "Acolyte, Initiate, Peon," etc.
Faction Data String 4 "FADT"
FADT Length Integer 4 Typically is 0xF0.
FADT Data Set of Integers 0xF0 [Not decoded yet.] However, this looks like a list of the attribute and skill levels required to attain each of the faction ranks, as well as the general disposition of other faction members to someone with that rank.
Faction Associations String 4 "ANAM"
ANAM Length Integer 4 Length of the other faction's unique name. --This is one of the few places where I've seen a string that isn't null terminated.
ANAM Data String Varies Unique name of another faction.
Disposition Modifier String 4 "INTV"
INTV Length Integer 4 Always 0x04
INTV Data Integer 4 Amount by which disposition is increased or decreased, if a member of *this* faction meets a member of *that* faction.

Race Data

 This is rare, but if you decide to create a new player or NPC race, this is what you would use. (Ex., Orc, Human, Elf.)
Race String 4 "RACE"
Race Size Integer 12 Length of the Race record.
Name String 4 "NAME"
Name Length Integer 4 Length of the unique identifier for this race.
Name Data String Varies Unique ID name of the race. Ex., "Ogre".
Friendly Name String 4 "FNAM"
FNAM Length Integer 4 Length of the friendly name, including trailing null.
FNAM Data String Varies User-friendly name for this race. Ex., "Ogre".
Race Data String 4 "RADT"
RADT Length Integer 4 Typically is 0x8C.
RADT Data Set of Integers 0x8C [Not decoded yet.]
Race Spell Information String 4 "NPCS" - This is a repeating field, and appears once for each spell, ability, or power the race has. These are spells that every member of the race will have by default.
NPCS Length Integer 4 The length of the NPCS field, always appears to be 32 (0x20), with extra space padded with nulls.
NPC Spell Name String Varies The name of the spell, ability, or power with trailing nulls to pad out any extra space.

NPC Data
NPC String 4 "NPC_"
NPC Size Integer 12 Length of the NPC record.
Name String 4 "NAME"
Name Length Integer 4 Length of the unique identifier for this NPC.
Name Data String Varies Unique ID name of the NPC. Ex., "npc_22".
Friendly Name String 4 "FNAM"
FNAM Length Integer 4 Length of the friendly name, including trailing null.
FNAM Data String Varies User-friendly name for this npc. Ex., "Joe da Killer".
Race Name String 4 "RNAM"
RNAM Length Integer 4 Length of the race name.
RNAM Data String Varies Name of the npc's race. One of: Argonian, Breton, Dark Elf, High Elf, Imperial, Khajiit, Nord, Orc, Redguard, Wood Elf.
Class Name String 4 "CNAM"
CNAM Length Integer 4 Length of the class name.
CNAM Data String Varies Name of the npc's class. One of: Acrobat, Agent, Alchemist, Alchemist Service, Apothecary, Apothecary Service, Archer, Assassin, Assassin Service, Barbarian, Bard, Battlemage, Battlemage Service, Bookseller, Buoyant Armiger, Caravaner, Champion, Clothier, Commoner, Crusader, Dreamers, Drillmaster, Drillmaster Service, Enchanter, Enchanter Service, Enforcer, Farmer, Gondolier, Guard, Guild Guide, Healer, Healer Service, Herder, Hunter, Knight, Mabrigash, Mage, Mage Service, Master-at-Arms, Merchant, Miner, Monk, Monk Service, Necromancer, Nightblade, Nightblade Service, Noble, Ordinator, Ordinator Guard, Pauper, Pawnbroker, Pilgrim, Priest, Priest Service, Publican, Rogue, Savant, Savant Service, Scout, Sharpshooter, Shipmaster, Slave, Smith, Smuggler, Sorcerer, Sorcerer Service, Spellsword, Thief, Thief Service, Trader, Trader Service, Warlock, Warrior, Wise Woman, Wise Woman Service, Witch, Witchhunter.
Alliance Name String 4 "ANAM"
ANAM Length Integer 4 Length of the faction name.
ANAM Data String Varies Name of the npc's alliance, if any. One of: Redoran, Thieves Guild, Temple, Imperial Cult, Imperial Knights, Hlaalu, Telvanni, Fighters Guild, Morag Tong, Ashlanders, Blades, Clan Quarra, Clan Aundae, Clan Berne, Camonna Tong, Imperial Legion, Mages Guild, Sixth House, Census and Excise, Twin Lamps, Nerevarine, Talos Cult.
Body Name String 4 "BNAM"
BNAM Length Integer 4 Length of the body name.
BNAM Data String Varies Name of the body type associated with this individual. Ex., "breton_m_head_04".
Hair Name String 4 "KNAM"
KNAM Length Integer 4 Length of the hair name.
KNAM Data String Varies Name of the hair style associated with this individual. Ex., "b_n_breton_m_hair_00".
Stats String 4 "NPDT"
Stats Length Integer 4 Length of the stats record section.
Stats Data Set of Integers NPC stats can be marked to "auto calculate," which means that they are based on the NPC's level and class. If auto-calc is turned on, then this NPDT section is much shorter than if all the stats are fully specified. Information is a set of 1-byte integers, listed in this order:
  • Auto-Calc (12 bytes long)
    1. Level
    2. [Not Sure; possibly high byte of level]
    3. Disposition
    4. Reputation
    5. Rank in Faction, with lowest Rank = 0
    6. [Not Sure] 0x6D
    7. [Not Sure] 0x64
    8. [Not Sure] 0x00
    9. [Not Sure] 0x00
    10. [Not Sure] 0x00
    11. [Not Sure] 0x00
    12. [Not Sure] 0x00
  • All Stats Specified
    1. Level
    2. [Not Sure; possibly high byte of level]
    3. Strength
    4. Intelligence
    5. Willpower
    6. Agility
    7. Speed
    8. Endurance
    9. Personality
    10. Luck
    11. Skills listed in ID order, with just the value of the skill given as a single byte.
    12. Health (2-Byte)
    13. Magicka (2-Byte, Spell Points)
    14. Fatigue (2-Byte)
    15. Disposition
    16. Reputation
    17. Rank in Faction, with lowest Rank = 0
    18. [Not Sure] 6 bytes of 0x00
FLAG String 4 "FLAG"
FLAG Length Integer 4 Always 4 bytes long?
FLAG Data Integer 4 This is a bitmask, containing values for several fields on the NPC.
  • First Byte:
    • 0x00: Male
    • 0x01: Female
    • 0x02: Essential
    • 0x04: Respawns
    • 0x08: Base Default Value
    • 0x10: Auto-calc Stats
  • Second Byte:
    • 0x00=Red Blood
    • 0x04=Skeleton Blood
    • 0x08=Metal Sparks Blood
  • Third Byte: Always zero??
  • Fourth Byte: Always zero??
NPC Spell Information String 4 "NPCS" - This is a repeating field, and appears once for each spell, ability, or power the npc has.
NPCS Length Integer 4 The length of the NPCS field, always appears to be 32 (0x20), with extra space padded with nulls.
NPC Spell Name String Varies The name of the spell, ability, or power with trailing nulls to pad out any extra space.
AI Data String 4 "AIDT"
AI Data Length Integer 4 The length of the AIDT field. Is 0x0C (12 bytes).
AIDT Data Integer 12 Various AI properties:
  1. Hello Chance
  2. 0x00 [Unknown meaning]
  3. Fight Chance
  4. Flight Chance
  5. Alarm Chance
  6. 0xF9 [Unknown meaning]
  7. 0x12 [Unknown meaning]
  8. 0x00 [Unknown meaning]
  9. 0x00 [Unknown meaning]
  10. 0x00 [Unknown meaning]
  11. 0x00 [Unknown meaning]
  12. 0x00 [Unknown meaning]
AI Wander String 4 "AI_W"
AI_W Length Integer 4 The length of the AI_W field. Is 0x0E (14 bytes).
AI_W Data Integer 14 The default wander AI module, generally associated with all npcs?
  1. Wander Distance (2 bytes)
  2. Wander Duration [Generally 0x05]
  3. Wander Time of Day (?) [Generally 0x00]
  4. ?? [Generally 0x00]
  5. Idle #2 Chance
  6. Idle #3 Chance
  7. Idle #4 Chance
  8. Idle #5 Chance
  9. Idle #6 Chance
  10. Idle #7 Chance
  11. Idle #8 Chance
  12. Idle #9 Chance
NPCO String 4 "NPCO" -- That is an uppercase oh not a zero. This is a repeating field, one for each thing carried in the NPC's inventory.
NPCO Length Integer 4 May vary, but appears to typically (always?) be 0x24, with extra unused space at the end of the NPCO record padded using nulls.
Item Quantity Integer 4 Indicates how many of that item are carried.
Item ID String Varies This the unique name/ID of the item being carried.

Creature Data
Creature String 4 "CREA"
Creature Size Integer 12 Length of the creature record.
Name String 4 "NAME"
Name Length Integer 4 Length of the monster's name, including terminating null character.
Name Data String Varies Unique ID name of the monster. Ex., "cave_rat".
Model Name String 4 MODL
Model Name Length Integer 4 Length of the model filename, including terminating null char.
Model Name Data String Varies Filename of the NIF file used for this monster. Ex., "r\Rust rat.nif".
Sound Name String 4 CNAM
Sound Name Length Integer 4 Length of the sound name, including terminating null char.
Sound Name String Varies Unique ID of the sound name, as defined in Morrowind. This is often the name of sub-directory under Sound/Cr that holds the .wav files used for this monster, but this is not guaranteed. Ex., "rat".
Friendly Name String 4 "FNAM"
FNAM Length Integer 4 Length of the friendly name, including trailing null.
FNAM Data String Varies User-friendly name. Ex., "Cave Rat".
Stats String 4 "NPDT"
Stats Length Integer 4 Length of the stats record section.
Stats Data Set of Integers 96 bytes ? (60 hex) Information is a set of 4-byte integers, listed in this order:
  1. Creature Type (0 = Creature; 1 = Daedra; 2 = Undead; 3 = Humanoid)
  2. Level (Rat = 2; Golden Saint = 20)
  3. Strength
  4. Intelligence
  5. Willpower
  6. Agility
  7. Speed
  8. Endurance
  9. Personality
  10. Luck
  11. Health
  12. Spell Points
  13. Fatigue
  14. Soulgem Value (i.e., number of item spell points granted. For Golden Saint, this is 400. For Rat, it is 10.)
  15. Combat Skill
  16. Magic Skill
  17. Stealth Skill
  18. Attack #1 Min Dmg
  19. Attack #1 Max Dmg
  20. Attack #2 Min Dmg
  21. Attack #2 Max Dmg
  22. Attack #3 Min Dmg
  23. Attack #3 Max Dmg
  24. ???
FLAG String 4 "FLAG"
FLAG Length Integer 4 0x04 0x00 0x00 0x00
FLAG Data Integer 4 This is a bitmask, containing values for several fields on the monster.
  • First Byte:
    • 0x00=No Movement
    • 0x01=Biped Motion
    • 0x02=Respawns
    • 0x04=Carries Shield/Weapon
    • 0x08=Always set???
    • 0x10=Swims
    • 0x20=Flies
    • 0x40=Walks
    • 0x80=Essential Monster
  • Second Byte:
    • 0x00=Red Blood
    • 0x04=Skeleton Blood
    • 0x08=Metal Sparks Blood
  • Third Byte: Always zero??
  • Fourth Byte: Always zero??
Scale String 4 "XSCL"
XSCL Length Integer 4 The length of the XSCL field, which looks to be 4 bytes always.
XSCL Data Float 4 The scale of the object as a floating point number from 0.0 to 1.0.
NPC Spell Information String 4 "NPCS" - This is a repeating field, and appears once for each spell, ability, or power the monster has.
NPCS Length Integer 4 The length of the NPCS field, always appears to be 32 (0x20), with extra space padded with nulls.
NPC Spell Name String Varies The name of the spell, ability, or power with trailing nulls to pad out any extra space.
AI Data String 4 "AIDT"
AI Data Length Integer 4 The length of the AIDT field. Is 0x0C (12 bytes).
AIDT Data Integer 12 Various AI properties:
  1. Hello Chance
  2. 0x00 [Unknown meaning]
  3. Fight Chance
  4. Flight Chance
  5. Alarm Chance
  6. 0xF9 [Unknown meaning]
  7. 0x12 [Unknown meaning]
  8. 0x00 [Unknown meaning]
  9. 0x00 [Unknown meaning]
  10. 0x00 [Unknown meaning]
  11. 0x00 [Unknown meaning]
  12. 0x00 [Unknown meaning]
AI Wander String 4 "AI_W"
AI_W Length Integer 4 The length of the AI_W field. Is 0x0E (14 bytes).
AI_W Data Integer 14 The default wander AI module, generally associated with all creatures?
  1. Wander Distance (2 bytes)
  2. Wander Duration [Generally 0x05]
  3. Wander Time of Day (?) [Generally 0x00]
  4. ?? [Generally 0x00]
  5. Idle #2 Chance
  6. Idle #3 Chance
  7. Idle #4 Chance
  8. Idle #5 Chance
  9. Idle #6 Chance
  10. Idle #7 Chance
  11. Idle #8 Chance
  12. Idle #9 Chance
NPCO String 4 "NPCO" -- That is an uppercase oh not a zero. This is a repeating field, one for each thing carried in the monster's inventory.
NPCO Length Integer 4 May vary, but appears to typically (always?) be 0x24, with extra unused space at the end of the NPCO record padded using nulls.
Item Quantity Integer 4 Indicates how many of that item are carried.
Item ID String Varies This the unique name/ID of the item being carried.

Leveled Creature
Leveled Creature String 4 "LEVC"
LEVC Size Integer 12 Length of the leveled creature record.
Name String 4 "NAME"
Name Length Integer 4 Varies.
Name Data String Varies Unique ID name of the leveled creature. Ex., "in_cave_alit_lev+0".
Data String 4 "DATA"
Data Length Integer 4 4 bytes
Data Flags 4 The first byte indicates whether monster placement should be "Calculate from all levels <= player's." A value of 0x01 means yes, a value of 0x00 means no.
NNAM String 4 "NNAM"
NNAM Length Integer 4 0x01
NNAM Data Short Integer Varies Percent chance none" value, stored as a single byte.
INDX String 4 "INDX" - Number of creatures referenced by this instance.
INDX Length Integer 4 0x04
INDX Data Integer Varies Number of creatures referenced by this instance. I.e. the number of CNAM/INTV record pairs.
Creature Name String 4 "CNAM"
CNAM Length Integer 4 The length of the creature's name, including terminating NULL. The CNAM/INTV records are repeated once for each creature that appears in this leveled instance.
CNAM Data String Varies Unique name of a creature.
Creature Level String 4 "INTV"
INTV Length Integer 4 0x02
INTV Data Short Integer Varies Difficulty level assigned to the previous creature (CNAM) record. This is stored as a 2-byte integer.

Activator

 For a reference to an Activator, see Reference to an Object.
Activator String 4 "ACTI"
Activator Size Integer 12 Length of the activator record.
Name String 4 "NAME"
Name Length Integer 4 Varies.
Name Data String Varies Unique ID name of the activator. Ex., "active_com_bed_01".
Model Name String 4 MODL
Model Name Length Integer 4 Length of the model filename for this activator.
Model Name Data Integer 4 Filename of the NIF file used for this activator. Ex., "f\Furn_Com_Bed_01.nif".
Friendly Name String 4 "FNAM"
FNAM Length Integer 4 Length of the friendly name, including trailing null.
FNAM Data String Varies User-friendly name. Ex., "Bed".
Script Name String 4 "SCRI"
SCRI Length Integer 4 Length of the name of the script, including trailing null.
SCRI Data String 4 Null-terminated name of the script used to activate this item. Ex., "Bed_Standard".

Miscellaneous
Misc String 4 "MISC"
Misc Size Integer 12 Holds the length of the miscellaneous record.
Name String 4 "NAME"
Name Length Integer 4 Varies.
Name Data String Varies Unique ID name of the misc item. Ex., "Misc_SoulGem_Common".
Model Name String 4 MODL
Model Name Length Integer 4 Length of the model filename for this item.
Model Name Data Integer 4 Filename of the NIF file used for this item. Ex., "m\misc_soulgem_lesser.nif".
Friendly Name String 4 "FNAM"
FNAM Length Integer 4 Length of the friendly name, including trailing null.
FNAM Data String Varies User-friendly name. Ex., "Common Soulgem".
Extra Data String 4 "MCDT"
Data Length String 4 Looks like it is usually 12 (0x0C).
Extra Data Values 12 This field holds the weight of the item in the first 4 bytes as a floating point number. The next 4 bytes hold the goldpiece value of the item as an integer. The last 4 bytes are by default zero: not sure what they are used for.
Script Name String 4 SCRI
Script Name Length Integer 4 Length of the script name for this item.
Script Name Data Integer 4 Unique name of the optional script that can be attached to this object. Ex., "make_them_sorry".
Inventory Picture Name String 4 ITEX
Picutre Name Length Integer 4 Length of the picture filename for this item.
Picture Name Data Integer 4 Filename of the TGA or other file used for this item when it is shown in the inventory window. Ex., "m\misc_soulgem_common.tga".

Container

 For a reference to a Container, see Reference to an Object.

To define a new container (for when you want to create a container that isn't empty, etc.):

Container String 4 "CONT"
Container Length Integer 12 Length of the rest of the container data.
Name String 4 "NAME"
Name Length Integer 4 Varies.
Name Data String Varies This is actually the unique ID of the container.
MODL String 4 "MODL"
MODL Length Integer 4 Varies.
MODL Data String Varies Name of the NIF model associated with the container. Example:
"o\Contain_Com_Chest_01.nif"
FNAM String 4 "FNAM"
FNAM Length Integer 4 Varies
FNAM Data String Varies User-friendly name of the container.
CNDT (Container Weight) String 4 "CNDT"
CNDT Length Integer 4 Always a length of 4.
CNDT Data Float 4 Container Weight. This is how much stuff the container can hold, as a floating point number.
FLAG String 4 "FLAG"
FLAG Length Integer 4 Length is always a value of 4.
FLAG Data Integer 4 By default is "0x08".
NPCO String 4 "NPCO" -- That is an uppercase oh not a zero. This is a repeating field, one for each type of thing inside the container.
NPCO Length Integer 4 May vary, but appears to typically (always?) be 0x24, with extra unused space at the end of the NPCO record padded using nulls.
Item Quantity Integer 4 Indicates how many of that item are in the container.
Item ID String Varies This the ID of the item that is in the container.

Sounds
Sound String 4 "SOUN"
Sound Length Integer 12 Total length of the sound definition, starting at the NAME field.
Name String 4 "NAME"
Field Length Integer 4 Length of the name, including null terminator.
NAME Data String Varies The unique ID of the sound. Ex., "Stone_Door_Open_01"
File Name String 4 "FNAM"
FNAM Length Integer 4 Length of the file name, including trailing null.
FNAM Data String 4 Filename of the WAV file used for this sound. Ex., "Fx\Stone01.wav".
Data Prefix String 4 "DATA"
Data Length Integer 4 Length in bytes of the sound data. Appears to always be 0x03.
Sound Data Set of Values 3 Not sure what these values mean. Most common values appear to be:
0xFF 0x00 0x00

Exterior Cell Data

 All exterior cells appear to be of the same size. They also appear to be arranged in a grid, with position 0,0 being on the equator in the center of the world map. Exterior cells are assigned a unique spot inside this grid, with some spots having negative values--haven't determined if negative indicates north or south of the center-line.

Still haven't determined where the "Illegal to Sleep Here" flag is stored. Nor do we know where the Map Color information is stored. The meaning of some of the internal fields is also a mystery, especially the PGRP, PGRD, and PGRC sections.

Begin Cell

 String 4 "CELL"
Cell Length Integer 12 Length, in bytes, of this cell, following the Cell Length.
Cell Name String 4 "NAME"
Cell Name Length Integer 4 Length in bytes of the cell name, including the required terminating null.
Cell Name String Varies The cell name, including the required terminating null.
Data String 4 "DATA"
Data Length Integer 16 Length in bytes of the cell data?
RGNN String 4 Name of the region this cell is associated with. The region controls the weather.
RGNN Length Integer 4 Length of the region name, including trailing null.
Region Name String Varies The name of the associated region, including trailing null.
NAM5 String 4 Start of NAM5 section.
NAM5 Length Integer 4 Length of the NAM5 string.
NAM5 String Varies? ???
Referenced Object List List of FRMR records Varies At this point, any creatures or other objects (including building exteriors) that show up in the cell are listed as FRMR records.
LAND Precedes the height map, vertex normals, texture coordinates, etc.
LAND Data Unknown 12 Unknown. Possibly the size of the remaining data.
Cell Position String 4 "INTV" (Integer Values?)
INTV Length Integer 4 Length of the INTV data, always 8.
INTV Data Int 8 This is the Row,Col coordinates of the cell. This might be its logical location inside the owning region, but more probably it is the cell's location on the world map. E.g., when you see an exterior cell, it often has a name like: "Ald Daedroth - (11,20)." The 11,20 shows the cell's position relative to all defined exterior cells, with values of 0,0 representing the equator/prime meridian, as about half of the cell values are negative.
Data String 4 "DATA"
Length Integer 4 Length of the DATA section.
DATA Unknown 4 Unknown
VNML String 4 "VNML"
Length Integer 4 Length of the VNML section.
VNML Data ??? ?? Probably vertex normal information, where each normal is evenly spaced in x,y terms across the land grid? Typically normals are stored as (X, Y, Z) vectors, so that is probably how they are stored here....
VHGT String 4 "VHGT"
Length Integer 4 Length of the VHGT section.
VHGT Data ??? ?? Vertex height field? Probably the Z coordinates of each vertex, since X & Y are spaced evenly in the cell. I'm pretty sure they're 16-bit values. The downside is that you're limited to 2730 feet above or below sea level (as the unit of measurement here is one inch?). The highest point of Dagoth Ur is only about 800 feet above sea level. There goes making Mt. Everest, eh?
WNAM String 4 "WNAM"
Length Integer 4 Length of the WNAM section.
WNAM Data Unknown Varies? Unknown
VCLR String 4 "VCLR"
Length Integer 4 Length of the VCLR section.
VCLR Data RGB Triplet List Varies Vertex Colors. RGB triples repeated for each vertex (1 byte per color per vertex, or 3 bytes per vertex).
VTEX String 4 "VTEX"
Length Integer 4 Length of the VTEX section.
VTEX Data Texture Index List Varies Vertex Colors. RGB triplets repeated for each vertex (1 byte per color per vertex, or 3 bytes per vertex).
PGRD String 4 "PGRD"
Length Integer 4 Length of the PGRD section.
PGRD Data Unknown Unknown Unknown
Data String 4 "DATA"
Length Integer 4 Length of the DATA section.
DATA Unknown 4 Unknown
Name String 4 "NAME"
Name Length Integer 4 Length in bytes of the name, including the required terminating null.
Name String Varies Appears to be a duplicate of the cell name?
PGRP String 4 "PGRP"
Length Integer 4 Length of the PGRP section.
PGRP Data Unknown Unknown Unknown
PGRC String 4 "PGRC"
Length Integer 4 Length of the PGRC section.
PGRC Data Unknown Unknown Unknown

Interior Cell Data
Cell String 4 "CELL"
Cell Length Integer 12 Length, in bytes, of this cell, following the Cell Length.
Cell Name String 4 "NAME"
Cell Name Length Integer 4 Length in bytes of the cell name, including the required terminating null.
Cell Name String Varies The cell name, including the required terminating null.
Data String 4 "DATA"
Data Length Integer 4 Length in bytes of the cell data.
Cell Data Set of Values 12 Default values are:

0x01 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x80 0x3F

The first four bytes area a bitmask that controls whether there is water in the cell and whether it is illegal to sleep here. (a) If there is water, the first value changes to 0x03. (Switch on the 0x02 bit.) Control the height of the water in the INTV data section (next). Not sure what the default 0x01 value means, though. (b) If it is illegal to sleep here, the first number should be 0x05. (Switch on the 0x04 bit). (c) If there is both water and it is illegal to sleep, change to 0x07 (first three bits set).

The next four bytes show the level of fog in the cell. Note: this information duplicates information also given in the AMBI data (follows the INTV section).

The last 4 bytes are a floating point number that is the Fog Density, in a range of 0.0f to 1.0f. Note: this information is duplicated in the AMBI data (follows the INTV section).

INTV String 4 "INTV" (Integer Value?)
INTV Length Integer 4 Length of the INTV data, always 4.
INTV Data Int 4 Height of water in the cell. For water to be turned on, Cell Data needs to be modified also (see above). E.g., for a water height of five units (not sure if this maps to logical game units of about 1 inch per, or what), do 0x05 0x00 0x00 0x00. Water height is given relative to z=0, which means that a "negative" water height will make sense in some cells.

AMBI String 4 "AMBI" -- contains ambient lighting information for the cell.
AMBI Length Integer 4 Length of the AMBI field data.
AMBI Data 4 Sets of 4 Integers 16 The default values are:

0x47 0x47 0x47 0x00 0xF2 0xD9 0xD9 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0x80 0x3F

The first 4 values correspond to RGB ambient lighting, although the default value of 0x47 does not correspond to what's shown for the default ambient lighting in the editor! 0x47=66, but the editor was showing 71 for these values.... But when you change the values, they match up exactly. The 4th value for ambient lighting always appears to be 0x00.

The next 4 values 0xF2 0xD9 0xD9 0x00 correspond to the RGB Sunlight values in the cell, with the 4th value always being zero.

The next 4 values (all zeroes by default) correspond to the RGB Fog values in the cell, with the 4th value always being zero. Duplicated in the Cell Data section above.

The last 4 bytes are a floating point number that is the Fog Density, in a range of 0.0f to 1.0f. Duplicated in the Cell Data section above.

NAM0 String 4 "NAM0" -- Note that is a trailing zero character, not an uppercase oh. It looks like this precedes the list of referenced objects (FRMRs) that are in a cell. This only appears once, no matter how many objects are in the cell.
NAM0 Length Integer 4 By default is 0x04 0x00 0x00 0x00.
NAM0 Data Integer 4 Not sure? At times, this appears to indicate the number of FRMR records in the list, but this isn't always the case....
Referenced Object List List of FRMR records Varies At this point, any creatures or other objects (including building exteriors) that show up in the cell are listed as FRMR records.

Reference to an Object

 Any time you reference any kind of object (static, container, creature, activator, etc.), you can use this same format. Only if you create a new kind of thing, or create a full copy of a thing, do you need to use a more detailed format.
FRMR String 4 "FRMR" -- Don't know what it stands for.
Field Length Integer 4 Value is always 4.
FRMR Data Index 4 Index of this object inside the cell. First object in the cell is 0x01. Next object is 0x02, and so forth.
Name String 4 "NAME"
Field Length Integer 4 Length of the name, including null terminator.
NAME Data String Varies The name of the referenced object. E.g., "in_v_s_corner01"
Scale [Optional] String 4 "XSCL" -- This field is only exported if the scale of the item is not 1.0.
Scale Length Integer 4 Always has a length of 4.
Scale Data float 4 The scale of the item, if not 1.0.
Lock Info [Optional] String 4 "FLTV" -- This field is only exported if the item is a container or door that is locked.
FLTV Length Integer 4 Always has a length of 4.
Lock Data Integer 4 The lock difficulty, a value of 0 to 100.
Key Info [Optional] String 4 "KNAM" -- This field is only exported if the item is a container or door that is locked and there is a key that opens it.
KNAM Length Integer 4 The length of the null-ternimated name of the key.
KNAM Data String Vaires The null-ternimated name (unique id) of the key.
Trap Info [Optional] String 4 "TNAM" -- This field is only exported if the item is a container or door that is trapped.
TNAM Length Integer 4 The length of the null-ternimated name of the trap.
TNAM Data String Vaires The null-ternimated name (unique id) of the spell effect associated with the trap.
Data String 4 "DATA"
Data Length Integer 4 Always 24 bytes long (0x18).
Data Set of Values 24

First 3 values are the X, Y, Z coordinates of the object, given as 3 floating point numbers (4 bytes each).

Next 3 values are the X, Y, Z rotation of the object, given as 3 floating point numbers (4 bytes each), but the numbers are stored as radians, rather than as degrees (although the editor shows them as degrees).

Teleporting Door

 For a non-teleporting door, see Reference to an Object.

Teleporting doors still appear as FRMR records inside a cell, not as separate records at the beginning of the file (unlike most other object definitions).
FRMR String 4 "FRMR" -- Don't know what it is
Field Length Integer 4 0x04 0x00 0x00 0x00.
FRMR Data Index 4 Index of this object inside the cell. First object in the cell is 0x01. Next object is 0x02, and so forth.
Name String 4 "NAME"
Field Length Integer 4 Length of the name, including null terminator.
NAME Data String Varies The name (ID) of this door. E.g., "door_cavern_doors10".
DODT String 4 "DODT"
Field Length Integer 4 Length of the DODT section: 0x18 0x00 0x00 0x00.
DODT Data Set of Values 24 The location information for where exactly you will arrive. I.e., the X,Y,Z coordinates and then the X,Y,Z rotation of the travel marker that is at the other end of the teleport.
Destination Name [Optional] String 4 "DNAM" -- Only used for teleporting to an interior cell.
Field Length Integer 4 Length of the destination name, including null terminator.
DNAM Data String Varies The interior cell name of the location where the door takes you to. E.g., "Balmora, Guild of Mages"
Data String 4 "DATA"
Data Length Integer 4 Length of the data: 0x18 0x00 0x00 0x00.
Data Set of Values 24 X,Y,Z location followed by the X,Y,Z rotation of the door itself. One floating point number for each coordinate.

Contributors:
Dave Humphrey
Dizzy
Erik Benerdal (aka Scarabus of Fudge Entertainment)
Jason Hoffoss
Jim Adam
Maurizio
Raymond Tukkers

To contribute information, please email Jim Adam.