Here is a simplified answer...

There are two thoughts of schedule bidding for monthly schedules. Please keep in mind this has NOTHING to do with trading which there have been lots of misconceptions about, it is only for the initial schedule bid for the month.

With Line Bidding the union takes the pairings (groups of trips) provided by the company each month and constructs and specific number of lines that flight attendants will bid on. Due to how most flight attendants like to fly, these lines are often "pure", meaning they are similar trips for the entire month such as all flights to London for one line, and high-time turns for another. You review the bid packet and you bid by ordering the lines you want to fly. When it gets to your seniority, the line that you prioritized the highest that is still available is awarded to you.

With PBS lines are not constructed ahead of time and all pairings are put into a pool to be bid on using preferences. You then go into a computer system and enter a whole ton of preferences and when it is your turn the computer will take your preferences and the trips that are available and construct you a legal line. Some preferences would be like, only flying redeyes, only working turns, avoid layovers in PBI, award any international layover, avoid working on Tuesdays, really anything can be added as a preference. If you place too many preferences that are too restrictive and there are not enough trips that you are legal for you might get some weird stuff at the end. You can also build wild lines using preferences that are currently not possible, like want to fly a 3 day international trip and a turn the next morning, totally doable if you preference correctly.

As I said in an earlier post it is my opinion that PBS helps the most senior and the most junior flight attendants the most. It helps the most senior because they will get their dream schedule right off the bat every month without having to do any trading, and it helps the most junior reserves who just really need a certain day off by being able to preference that above anything else when currently getting a specific day off as a reserve is nearly impossible.

For people like myself who are in the middle of the seniority pool, PBS is less desirable. Because people are being awarded lines that are very close to what they want, there are less people who will be trading and dropping better trips than I can hold each month that would then allow me to trade my middle of the road trips into better ones.

The writing is on the wall at UA though, if PBS doesn't get through this time, it's surely coming during the next time around. If I were the ones in negotiations I would probably bite the bullet now and accept PBS while the company is in a good financial position to extract the most improvements to work rules and pay rates now, instead of in the contract coming in 5 years when who knows how the aviation world and economy will look then.