If the driver is unassigned or cover and management either doesn't have work in the home center or another center is really hard up for guys they can work the driver out of the other center if it's in the same bargaining unit. The driver has the right to report to their home center though at regular start time and drive a spare package car to the foreign center or get paid mileage.
I pulled routes in 3 centers while I was cover. The cool thing was a sup usually offered to drive as they know the area so you are essentially a driver helper for the day making driver pay. Nice easy relaxed day... Sups usually bought my lunch and the loads were always magically light so I got done in 8 hours or less then got paid to drive back to home center.