Originally Posted by TodKindrsChikun

That’s somewhat incorrect. Secondary vacancies occur when the staffing for a BES(base, equipment, status) falls below a min level set by the company as a result of pilots bidding off that airplane. The company may backfill up to the min staffing level via a secondary bid that runs concurrent with the primary bid. The secondary bid is open to all pilots and not just pilots junior to the ones bidding off as the poster alluded to. Sometimes secondary bids for certain BES can go very senior if there are only a few awarded.

8-C-4 of the UPA