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Official game expansions ("downloadable content", known as "DLC") typically are provided as ESM files along with other "assets". The ESM files are loaded first, and are considered "masters" to the ESP files that depend upon them. There is generally only one ESM file, but there may be any number of ESP files (including none). "Plugin" files have one of two possible file extensions: ESM and ESP.
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The "plugin" files are the only ones that appear in the "load order", which determines the sequence in which the game engine loads them into the game from the "top" (lowest numbered) to the "bottom" (highest numbered) position in the sequence. Within a "mod" package can be a number of "asset files" which add to or replace the existing vanilla assets (meshes, textures, sounds, animations, XML files, etc.), and one or more "plugin files" that tell the game about the existence and use of the new assets and where they are placed in the game. A "mod" is a package (archive) of related files that make some change to the "vanilla" (unmodded as delivered by the publisher) game.
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