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This article is an introduction to mod files: their uses, differences, and importance both to using and making mods. Understanding how mod files work is fundamentally important to understanding how to mod.
Master Files (*.esm) and Plugins (*.esp)
Master Files (files with .esm - ie, Skyrim.esm) and Plugin files ("TES files" or "mods" for short - files with .esp - eg, SkyrimMod.esp) are the core data files of Skyrim. A master 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. The Creation Kit is the tool we use to view and edit master files and plugins.
Master files and plugins are largely identical in format, but have some important distinctions in practice.
- A master file is autonomous; it relies on no information other than itself. The Creation Kit will not create master files.
- A plug-in (ESP) file relies on information from a Master ESM file. Plug-ins CANNOT refer to information in another plug-in. They can only refer to data from a master file (though they can refer to multiple master files).
Note that not all game data is contained in the master and plugin files. Textures, meshes, sounds, videos, etc. are all part of the full game, and are stored separately in Archives (.bsa files).
Master files, plugins and savegames are usually dependent on other master files or plugins (the exception being the original master file that shipped with the game - that master file has no dependencies, and runs on its own). Typically, a Skyrim mod will be dependent on the original Skyrim master file. The masters of a given mod or savegame is simply the list of "parent" mods that the mod or savegame depends on. The masters of a mod can be seen in the Creation Kit details view.
Masters lists are important because they help identify where objects in a mod or a savegame come from. I.e. when a savegame remembers an object, it essentially remembers it as something like "the 2,034th object defined in my fourth master". If the master list were somehow to be erased, then the savegame would lose the ability to match the object in the player's inventory with original object defined in the mod.
In regular gameplay, it's not really necessary to know about masters, but modders need to understand them, since gameplay debugging often requires working with FormIDs, and FormIDs are the way the game encodes information like "the 2,034th object defined by my fourth master".
Records and Overrides
As mentioned before, a mod file (master file or plugin) is basically a database of all the objects in the gameworld. To be more specific, the mod file is a collection of records. The records in a given mod are mixture of 1) new records, and 2) modifications of existing records (i.e. of records defined by the mod's masters). E.g. "Cool Swords.esp" might introduce three new swords, as well as modify two existing swords defined by Skyrim.esm. Each of these swords is represented by a single weapon record (so 3+2 = 5 records in total).
The important thing about records is that each record is a single indivisible object as far as Creation Kit and the game engine are concerned. Think of the game world as a table with stacks of playing cards. You can add a new card, or you can override an existing card by putting a new card on top of it. So, in the example above, the three new swords form three new stacks, and the two modified swords go on top of existing stacks overriding the original cards/swords/records.
Now, suppose that there's a second mod "Heavy Swords" which adds two new swords and redefines four swords from Skyrim to be more "heavy". Suppose that of these four overridden swords, one conflicts with an override done by "Cool Swords" -- i.e. they both try to change the same sword.
Again, this is just like the card game. The two new swords form two new stacks, and the four redefined swords go on top of existing cards, overriding them. But in this case, one of those cards goes on top of the card added by Cool Swords-- thus overriding its changes as well as as original Skyrim.esm sword/card. Note in particular, that the changes are not merged. Each newly added card/record completely replaces the card/record under it.
The card game above is why load order is important. Load order determines the order in which mods lay their cards down on the table. Skyrim goes first, then the next mod in load order, up to the last mod. Whenever two mods conflict, the later loading mod will win.
As far as the order is concerned, master files (.esm) always load before plugins (.esp) Then within each groups, files are ordered by modified date.
So the following load rules exist:
- Plugins always trump master files
- Newer plugins trump older plugins
- Newer master files trump older master files
To change load order, change the modified date of the files.
Remember the the game engine is effectively playing a game of stacked cards. And when you create a mod, you're simply creating a hand of cards that will be played in that game. From that ending table, with its various stacks of cards, arises the gameworld that the player enters and lives in.
When working in the Creation Kit, you can load multiple plugins, but you need to set one to be 'Active'. When you save, your changes will be written to the active plugin.
Some notes on Active files:
- You can only make a plug-in (esp) file active. You cannot save changes to a master file.
- The active file is always loaded last in the editor, regardless of its file date. This guarantees that all the info and changes in the active file can be viewed without being overwritten by another plug-in.
- If you do not set an Active plugin, you will be prompted to create a new one when you save. This then becomes the active file.