Seniority prevails. If a dispatch is made to a junior driver before a senior driver that is a grievance. As long as they are both at the terminal at the same time. Of course, some don't care. So it is up to the particular person weather they feel cheated by the particular grievance.
Say Mr.dispatch has a buddy and he likes to give him gravy everyday; well if he gives him a dispatch, before a senior man and the senior man wants that dispatch, he can request it, per seniority.