In long past years, a higher seniority driver (reg ft or cover) had no trouble passing on a workday, so long as there was another driver to take their place.
About 5 years ago, UPS changed their view on this and basically said, if you are on the schedule, unless you want to burn an optional or were taking a sick day, you were going to be working.
They figured, it was technically possible to save a month of healthcare premiums by starving out a low seniority driver and keep that off their bottom line. Here, you need 40 hours per month to get health bennies.
There were some months they kept the lowest driver down to only one or two days.