in the official guide book (prima - free with red hammer) it mentioned that you could link the sentry and guard waypoints to a nice affect - sentry units would walk around as scouts looking for enemy, but then run back to the guard units and then the guard units (maybe tanks) would go off to the possition where the enmy were sited.
also:
Guard - great waypoint for peeps who guard bases - if any unit comes under fire within a certain range of a guard unit, they will head towards where the enemy were sited and attack.
Sentry - units will wait for enemy, attack any1 they see, and then return to sentry possition untill the next engaement