In the Southern Supplement, we have FT swing drivers which are FT drivers without a bid route. These drivers will cover for vacations, call in, and days off, there is always work available to them, as management makes sure not to staff more FT swing drivers than needed.
The next sort of driver is a PT cover driver, who is also used to cover the aforementioned. As far as work being available, generally speaking about half work every day, Wendsday most work if not all. When not enough work is available, they go back to working their normal PT shift.
Now in our building, we have enough FT bid drivers who apparently are set for life and don't want to work every day, so this gives more opportunity for PT cover drivers to work. I've seen some of these drivers almost come to blows with each other on who's going home. Not sure how your center is, but that's how it is in Roswell.