Data File

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File Extension: .esm, .esl, .esp, .fos, .fid, .fud, .fvd, .tes, .ccc

A Data File acts as a database of all of the data for the world, including object data, dialogue, gameplay settings, object placements, AI settings, landscape, Cells, etc. There are several kinds of files associated with data files. Masters, Light Masters, and Plugins are largely identical in format, but have some important distinctions in practice. Other files associated with game data include Game Saves, Version Control meta, and temporary files generated by the Creation Kit.


The Creation Kit will not create master files without enabling the Creation Kit's built in Version Control system. The version control merging process takes the interim ESP data and merges it into a shared master ESM file. The shared ESM is used by collaborators to create their own interim ESP data. Some community made tools exist which are capable of merging ESP files to masters in a similar manner as Creation Kit version control system as well.

Usually, most authors who want to create ESM files will create an ESP file for editing in the Creation Kit. Once the ESP file is ready for public release, they will manually convert the file meta and extension to an ESM master. The ESP should be excluded from public releases when the intention is to release a master file.

The ESP may be kept by the author indefinitely, or manually converted to and from ESP as needed.

Light Master

The Light Master (.esl) file is a new optimized file format introduced in the Fallout 4 v1.10 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 it's maximum capacity for form records.

A Light Master file cannot be edited in the Creation Kit. Active development should be done in an normal *.esp plugin file.

Light Master Notes

  • Light Master files released by Bethesda Game Studios (Creation Club releases) are flagged as master files and always load immediately after the official Bethesda masters, in the order specified by the game's .ccc file. These .esl files cannot be deactivated.
  • Light Master files released by modders are flagged the same as ESP files, but these also load with the master files, in the order specified in plugins.txt. These .esl files will need to be activated before the game will load them.
  • Light Master files created by modders cannot use standard ESP files as masters or this will cause the ESP file to also load before the .esl in the master files section. This is new behavior for the game engine as of Skyrim Special Edition v1.5.3.
    • Any dependencies of the normal plugin a light master depends on are not loaded early.
    • Creating light masters that are dependent on normal plugins is highly discouraged.

Light Master Budgets

A Light Master file has a budget of no more than 2,048 forms, with Form IDs 0x800 to 0xFFF available for use. A absolute maximum of 4096 ESL files can be loaded by the engine at once, but in practice this is difficult to achieve. 331 plugins can be loaded that each contain 2048 ALCH records, but 970+ plugins can be loaded that each contain one CELL record and 2047 REFR records. There may also be a file handle limit, and a reference limit that prevent 4096 ESL files from being loaded.

Light Master Runtime

Light Master files are merged into plugin slot 0xFE at runtime. Their runtime Form ID is 0xFE[000](Hex Load Order)[000](Original Form ID).

Form 0xFA0 from the 10th loaded ESL file would be 0xFE00AFA0 at runtime. From the 200th ESL file, it would be 0xFE0C8FA0.

Light Master Publishing

To publish an (.esp) plugin as an (.esl) light master, load your ESP file, and under File, select Convert Active File to Light Master. It is good practice to run Compact Active File Form IDs to optimize your Form IDs within the 2,048 form budget before converting. Compacting Form IDs does also change Form IDs in the ESP file, so be sure do this before converting to ESL, and be sure to save the ESP file afterwards.

Do not compact Form IDs on an ESP file you have already released and are updating. Because Form IDs change, you essentially create a new plugin when compacting. This new plugin will not be compatible with saves using the old version.

Light Master Updating

Updating ESL files has more restrictions than updating ESP files, but not many. Because of the 2,048 Form ID limit, it is important to be careful when deleting forms not to reuse Form IDs. A saved weapon form could be deleted in your plugin, then have an armor form take the Form ID when compacted.

Be sure to save ESP files after running Compact Active File, because otherwise compacting again with new forms might give different Form IDs for forms already in a game save. As long as development is done on compacted ESP files that match released ESL files, forms can be added under the 2,048 form limit, and existing forms can be edited without issue.


A plugin ESP file created in the Creation Kit may only rely on data from itself and any master ESM files. It's also possible to create plugins that depend on other plugins using third-party tools.

  • Plugins are not required to contain any data.
  • Plugins are not required to have a master if they are empty or add new content.
  • Plugins can only refer directly to data from a master file, though they can refer to multiple master files.
  • Plugins can only refer to information in another plugin indirectly by using scripts.

Light Plugin

Also known as "ESL-Flagged ESP" or "ESPFE", this is a combination of a light master and a plugin. It's a file with the .esp file extension, but the ESL header flag.

  • Like light masters, light plugins have a budget of at most 2048 forms, between 0x800 and 0xFFF, and are merged at runtime into the 0xFE slot.
  • Light plugins are loaded together with regular plugins. This means, they can both override other plugins, and be made to require plugins as masters without negatively affecting the load order.
  • In certain circumstances, regular plugins can be made light without breaking compatibility to existing saves.

A light plugin can be created using various means:

  • Use the Creation Kit to create a light master as described above. Then, change the resulting ESL file's extension to .esp
  • Load the plugin in question in xEdit
    • Once loaded, right-click the file, select "Compact FormIDs for ESL".
    • In the Header section, add the ESL flag.
    • Save the file.
  • Vortex is capable of making plugins "light" from it's Plugins tab.

Game Save

A Fallout Save (.fos) is used to restore the games state between sessions. The Creation Kit cannot create, edit, or view save files. Uninstallation of any data file from a game save is not recommend, and officially not supported.

Removing Master Files

While ESP files can sometimes be uninstalled from game play without consequence, an ESM file can never be removed in this way. In short the Creation Engine refuses to remove an ESM from a game save.

However, this can be overcome by modifying the game save meta to think the missing ESM was actually an ESP. Once the master list has been edited on the game save file header, the game engine will happily remove it. Wrye Bash provides a game save editor capable of disabling masters.

Version Control

.fid, .fud, .fvd
The FID, FUD, and FVD files are generated by the Version Control system. These files are used by the Creation Kit to track and merge changes between file versions.

Modifying Other Mods

While the Creation Kit allows ESPs to use and modify ESMs, it will not allow similar actions for ESPs. The basic problem here is that the Creation Kit refuses to allow an ESP to become a master of an ESP. And without that master relationship, it's impossible for the new ESP to refer to anything in the old ESP (and thus is unable to use or modify it).

However, it's possible to trick the Creation Kit into remembering another ESP as a master. Once this is done, then the new mod is able to use and modify the old ESP (almost) as if it were a regular ESM file. See De-Isolation Tutorial link at the bottom of this page.

Also of note is that ESMs can modify other ESMs -- within certain limits. The main limit seems to be that one ESM cannot add items to worldspaces defined by another ESM. Or rather, it can't do so safely -- since doing so causes landscape and buildings to disappear during gameplay.

Data File Conversions

The Creation Kit does not provide a straight forward conversion between data files, though it does have some limited methods for conversion. Several community made tools also exist which allow easy conversion between ESM, ESL, and ESP files.

Light Master to Plugin Conversion

While not directly supported, an ESL can be converted back into an ESP for direct editing.

As of Update v1.10+, simply rename the ESL file extension to *.esp. With the renamed file extension, the Creation Kit will load the ESL as a standard ESP plugin. When Active Plugin is saved, the Creation Kit will complete the conversion by removing the master header flag. The ESL is now converted into an ESP plugin file.

The Convert Active File to Light Master command may be used to publish the file as an ESL light master again.

Light Master to Plugin Manual Conversion

Before Update v1.10, manually converting a light master to a plugin was not supported by the Creation Kit. To manually convert a light master to a plugin, do the following.

  1. Rename the file back to .esp and open it for editing with xEdit,
  2. In the Header section, edit the flags to trigger a re-save. Simply choosing the ESM flag on and off again will suffice.
  3. Save the modified plugin. It will then be a binary identical to the original *.esp it was created from.


  • The Creation Kit is the primary tooling for working with data files.
  • xEdit
  • Wrye Bash
  • There are other community software projects that provided tooling for working with data files.


  • By tradition, the file extensions stand for Elder Scrolls Master and Elder Scrolls Plugin.

See Also