Record file

A record file is a game data file used in the Gamebryo and Creation engines. Record files contain information about in-game items, characters, world spaces, and settings. Game data such as textures, models, sounds, and other assets are found in BSA files rather than record files. Record files come in two forms: master (ESM) files and plugin (ESP) files. Both master and plugin files are found in the Data subfolder of any parent game directory (i.e. where the game's .exe file is found).

Record files can be viewed using the GECK or Creation Kit, or with third-party tools like xEdit.

Master file
A master file, using the .esm file extension, (also called an ESM file, from Elder Scrolls Master) serves as the primary database of game files which plugin files can reference, modify, or add to. They are loaded prior to plugin files.

ESP
Plugin files, using the .esp file extension, (also called an ESP file, from Elder Scrolls Plugin) are largely identical to master files in structure but are loaded after them, and are unable to modify other plugin files. They are intended for use with player mods, as base games and their DLC are both packaged with .esm master files.

ESL
The Light Master file, using the .esl file extension, is a new optimized file format introduced in the Fallout 4 patch 1.10 update. Using Light Master files will allow more plugins to be loaded than the previously possible due to the 8-bit plugin ID limit of 255. A Light Master does this by using more of the Form ID range for load order, while also sacrificing its maximum capacity for form records. It is primarily used for Creation Club content.

Load order
The load order of record files determines when changes the files make are applied. Files appearing lower in the load order will overwrite changes made to records by files earlier in the load order.

Record
Records are the individual structures of information that make up record files. Each record contains a record header, with information such as a signature, Form ID, Editor ID, data size, flags, and version control information.

Record signature
The signature within a record's header is what denotes its type. Some common record signatures are shown below:

Fallout 3

 * Fallout3.esm (base game)
 * Anchorage.esm (Operation: Anchorage)
 * ThePitt.esm</tt> (The Pitt)
 * BrokenSteel.esm</tt> (Broken Steel)
 * PointLookout.esm</tt> (Point Lookout)
 * Zeta.esm</tt> (Mothership Zeta)

Fallout: New Vegas

 * FalloutNV.esm</tt> (base game)
 * DeadMoney.esm</tt> (Dead Money)
 * HonestHearts.esm</tt> (Honest Hearts)
 * OldWorldBlues.esm</tt> (Old World Blues)
 * LonesomeRoad.esm</tt> (Lonesome Road)
 * GunRunnersArsenal.esm</tt> (Gun Runners' Arsenal)
 * CaravanPack.esm</tt> (Courier's Stash)
 * ClassicPack.esm</tt> (Courier's Stash)
 * MercenaryPack.esm</tt> (Courier's Stash)
 * TribalPack.esm</tt> (Courier's Stash)

Fallout 4

 * Fallout4.esm</tt> (base game)
 * DLCRobot.esm</tt> (Automatron)
 * <tt>DLCworkshop01.esm</tt> (Wasteland Workshop)
 * <tt>DLCCoast.esm</tt> (Far Harbor)
 * <tt>DLCworkshop02.esm</tt> (Contraptions Workshop)
 * <tt>DLCworkshop03.esm</tt> (Vault-Tec Workshop)
 * <tt>DLCNukaWorld.esm</tt> (Nuka-World)