This has always been an issue at UPS. I worked there 31+ years before retiring, and unfortunately the contract doesn't specifically have concrete language on this issue.
I'm in NorCal and for the first 10 years at my center, it was a free for all bid. Damn near everyone would sign the bid. Then they'd come around one by one and ask, are you interested. If you passed, you were out. If you accepted, you had a one day ride with a supe, and two days on your own to try it out. Worked well.
We get a new center manager while a bid list is up, and I'm high seniority on the list. New center mgr. says route is yours. I say, nah, I'm going to pass, he says no, you bid you take. He comes from a center still within our local, so I'm now finding out that there is no consistent policy on this. I argue it, say past practice should be in effect, and win. Keep my original bid route.
Years later, they keep the anyone can bid and pass, but kill the supe ride, three day tryout still in effect. Then a few years later, anyone can still bid and pass, but the three day tryout is gone. That's where it remains as far as I know.
At every union meeting about contract negotiations I would bring up this issue of not having a consistent policy regarding this in the contract language, and every contract it was avoided.