From Quake Wiki

Adds two engine-called aliases named startmap_sp and startmap_dm which are called when the engine tries to start a singleplayer game from the menu (startmap_sp) or the -listen or -dedicated options are used or the engine is a dedicated server (uses startmap_dm). These allow a mod or game to specify their own map instead of start, and also distinguish between singleplayer and -listen/-dedicated, also these need not be a simple "map start" command. They can do other things if desired. Startmap_sp and startmap_dm both default to "map start".