Instead of the company manually building each line and pilots bidding on the lines they want, a computer program has all the trips in one giant pool and starting at pilot #1 looks at what bid preferences they want (days off, start/end time, layover city, etc) and build them a line. Then it moves on to pilot 2, 3, etc. PBS is great if you're senior because you can get pretty much any line you want, especially if you need an odd set of days off or something where with line bidding you probably wouldn't have a line that suits your needs. If you're junior it sucks because you're stuck with all the trips that wouldn't work on anyone else's line and the program is forced to cram your schedule with whatever works, regardless of your bid options.