ExMx
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In video games, "ExMx" (alternatively, "ExLx") is a common format for naming levels and the corresponding file names. The "E" and "M" mean "Episode" and "Mission" or "Map" respectively. ("L" stands for "Level".) The numbers put in place of the "x"s identify the exact level in the game. For example, "E2M3" denotes the third level of the second episode.
One of the first games to feature this level identity scheme was Wolfenstein 3D. Later, other games like Doom, Duke Nukem 3D, and Quake adopted it for simple level identification.
Similarly, the scheme "MAPxx" is sometimes used to identify levels when they are not divided into sets of episodes. This identity code was first used by Doom II. Other games like HeXen adopted this code for identity for single episode games.
Other games, for instance the Unreal series, use more conventional names.
