The ESM file format stands for Elder Scrolls Master file. It has been in use since The Elder Scrolls III: Morrowind, and is used in conjunction with the ESP and BSA file formats. All ESPs require an ESM file in order to work. For example, Skyrim ESM file is labeled Skyrim.esm, and is required to run all mods for The Elder Scrolls V: Skyrim.
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