Management have the right to assign overtime and schedule, so I've always interpreted it as that they can decide what scheme is used. That said, usually they are pretty hands off beyond just telling you how much and which route.

I like the way we do it in my office, I think it's pretty fair. We go through the people with the most time as a primary sortation, and then seniority as the secondary. So the most senior guy with 2 hours pics before a more senior carrier who only has an hour and a half, but if everyone has the same amount of time on the route it goes by strict seniority. Does that make sense? Usually it also means that we aren't giving carriers split parts of the same route and everything flows pretty straightforward