File with .DEM extension contains recording of gameplay of FPS (first person shooter) type game running on Source engine. Footage is taken either from single- or multi-player game mode session. DEM file can be opened only with the game with which the recording was created. This files are usually created with the intention of sharing gameplay recording with other users. Games like Counter-Strnke, Hexen II, Quake or Unreal Tournament all support this file format. Playback of DEM files offers such functions as camera angle control, playback speed control and music editing.

DEM files are supported by Windows operating system. In order to play the file user must place the file in game’s installation folder, start the game and then open the console window. Next step is to input ‘viewdemo’ command into the console, followed by name of the file. DEM files can be converted into movie format by using VirtualDub software, which replays the file frame by frame and saves it in chosen output format such as .AVI. There is a size limitation of 2 GB for AVI videos created by Valve's Source engine. User can install QuickTime component to encode the .H264 codec in order to bypass the size limitation.