A NAV file is a Navigation Mesh file for Counter-Strike: Source bots, Left 4 Dead bots, Left 4 Dead 2 bots and Team Fortress 2 bots and NPCs. It defines, among other things the walkable space in a level. The file is generated automatically when bots are played for the first time on a specific map. One can edit a Navigation Mesh manually by typing nav_edit 1 in the console with sv_cheats enabled. NAV creation is an open-source task available in the SDK. If one were to implement their bots, it would be easier because NAV creation is already supplied.