A function included in the GECK version 220.127.116.11.
This is one of 4 functions which can be used to increase the game's performance in situations where many actors are in combat or otherwise doing complex pathing. Note that the flag is cleared every frame, so this function needs to be called in a script running on a reference in the loaded area.
When the SetAllReachable flag is set, actors do not use pathing. They assume they can always move in a straight line from where they are to where they want to go.