A game's identity key is a text string used to uniquely identify a game “mode” playable with the Doomsday Engine. Game modes are registered by the installed game plugins during startup.
A complete listing of all supported games and their associated IDs can be found in the Readme.
A single game component, such as the Doom Plugin, may implement multiple game modes. Each mode emulates a specific version of the original game and typically has its own IWAD file. In some cases, several modes may utilize the same resources as other game modes.