SWA figures the person who is most concerned about a pilot's schedule is not the schedulers but the pilot. Therefore the union in conjunction with the company have developed different strategies/policies/agreed upon contractual procedures that allows pilots to (1) trade trips with other pilots (2) trade trips with the company for other trips in a pool that are unfilled by the company for various reasons (trip pulls for military guys, vacation, etc) (3) post trips for all pilots to see & that any pilot can pick up & not necessarily have to "trade" their trip with you to get your trip...a straight giveaway .
In the strick sense of the word it can be "preferential trading" not bidding since this all occurs after one gets one schedule. It is more like "fixing" one's schedule or fine tuning it to make one's schedule more productive or increase the density of work while away or picking up trips up to the FAA maximum, something regular lines don't do obviously. PBS involves putting one's preferences down & then the computer builds lines based upon these request/preferences & seniority. The improvement of the lines for our 4400 schedulers/pilots is sometimes based upon seniority but other features don't require any seniority to accomplish the goal.
With the flexibility & with some or little seniority pilots can improve their pay, time off, amount of work done or whatever one's goal is in the month they are flying....being able to write lines for that many differences/variables would be tough....trading for it & being motivated to fix one's own schedule & having the tools to do that allows the company schedulers to do what they need to do, fill the holes & keep the airline flying when things come up. The rest of us will keep mama happy by adjusting our schedule accordingly. Hope that helps explain a little of what our scheduling system is like. Still would be greek to me if I was on the outside looking in but hope that helped a little.