When I called crews on the Milwaukee Road in the 1970's, we had assigned jobs (yard jobs and locals) that were bid on seniority. Road jobs were called on pools operating between specific terminals. We also had an extra board that was controlled strictly by seniority. However, no one had to show up for the extra board unless they were actually called to work.
I worked under an old-time yardmaster (he was probably younger than I am now) who told me he was once 38 times out on an extra board, so he went fishing. Darned if he didn't miss a call! You see, anyone senior on the extra board could refuse a call as long as there was someone below them on the roster who could be called. If everyone senior to the last man on the board turned down a job, and the most junior man could not cover the job (missing, sick, drunk), then the jobs would be forced on the next person above him (who had already turned down the call). In other words, once the jobs had been turned down in seniority order, they were forced in reverse seniority order.
I once had this work to my advantage. In 1973, I had very little seniority, but more than employees who gave up their seniority to return to college. I had been bidding on positions but did not have the seniority to be awarded one. Finally, a job came open that I did not bid on, not because I didn't want it, but because I was too discouraged to bid on it. As the bottom person on the seniority roster, I was forced on it. And it turned out to be a pretty good job!