Following are the goals the PBS core is trying to achieve.
PBS has to award lines without conflict with carry-in and pre-defined activities while honoring all government and company/union requirements, and then honors the preferences.
[Honor Preference by Seniority] (strict seniority, NOT fair equity)
This is the most important part of the PBS. PBS may be referred to as "Preferential Bidding by Seniority". Unless being awarded to senior crewmembers, all pairings are available to each crewmember to make the best legal line based on his/her preferences. Seniority is honored as long as there are no pairings more preferred and legal to cover yet awarded to a more junior crewmember.
A PBS should leave no open pairing or not too many open pairings (of certain seat/language/equipment) on any given day and should not leave too many overall open pairings. It should produce targeted line holders and reserves. To achieve this, PBS must have the ability to deal with situations where crewmember's preferences cannot be fully satisfied while honoring company goals.
[High Overall Satisfaction]
The satisfaction of the junior crewmembers is a key difference between different PBS algorithms while all PBS' know how to satisfy senior crewmembers. A reasoning report should be provided for all declined preferences.
[Finish in Time]
PBS should finish and publish the results of all bid packages in time. Non-related bid packages should be able to solve at the same time; while related bid packages have to solve in sequence in order to handle preferences such as fly avoid, fly with, avoid check airman, etc.