For normal bidding, FedEx uses schedules built by a computer, reviewed by the union and adjusted as needed by a negotiation process. For those that bid "Custom lines" (known at FedEx as VTOs for line holders or Custom lines for those just activated) , then you can say that a PBS type system is used. But the VTO lines are only built using trips that were either disputed by the union; dropped for vacation or training; or not used in the line building process.
I have friends at Northwest (Delta) and they almost unanimously agreed that PBS benefits those with seniority in each seat. If you are in the middle, there's a chance that some of your desires would be fulfilled. If you are at the bottom, a nightmare.