No, it is the talkgroup used, usually instead of, any of the mutual aid channels during a high-traffic event where the users do not wish to tie up another group (such as Sheriff or EMS channels). I rarely ever hear the mutual aid channels used here for mutli-agency events.
Ok... odd use as with a wide area system it isn't really needed - I'll add it tomorrow as soon as I can figure out how to word it so the use is clear as mud