rules regarding afl players...can someone explain ?

i've noticed that glenelg have named james sellar for this weeks clash with north at the bay, which leads me to this question via a statement, at the end of 2006 both tom logan & ryan willits were not allowed to play for glenelg in the last minor round game because they had played afl the week before which although ludicrous is the rule so this week sellar is allowed to play yet those other 2 players were not which i don't understand what the difference is & 2nd seeing sellar is allowed to play why couldn't all the other players (who are not at the mcg) be allowed to play for their sanfl teams ?
i'm a confused bayman
i'm a confused bayman