Use of the Term "secondary" has been used at both legacy carriers well before the merger. Not sure where the term "backfill" came from, except for a process, but I didn't see it in either prior merger contract related to equipment bids. It could have been made up by someone unknowingly.
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Current CBA
8-C-4 Awarding of Vacancies Upon closing of a vacancy bulletin, pilots shall be awarded the number of advertised primary vacancies. At the same time, pilots may be awarded secondary vacancies that have become available due to the awarding of the advertised primary vacancies. Further, awards may be made to fill the vacancies resulting from all subsequent awards. Secondary vacancies shall be awarded to ensure the published minimum number of positions are maintained for all Categories. These secondary vacancies shall not have been advertised, but shall share the same effective date as the primary vacancies.
UAL 2003/2007
8-C-4 Awarding of Vacancies Upon closing of a vacancy bulletin, pilots shall be awarded the number of assignments needed in each of the advertised vacancies. At the same time, pilots shall be awarded assignments, if needed, in domicile, status and equipment types which have become available due to the assignment of pilots to fill the advertised primary vacancies. Further, awards may be made to fill the vacancies resulting from all subsequent awards. These secondary vacancies will not have been advertised, but will share the same effective date as the primary vacancies.
Cal 2002
D. Displacements may occur on any System Bid award. Adjustment Bids 1. A Pilot Bulletin will be issued announcing an Adjustment Bid at least five (5) Calendar Days prior to an Adjustment Bid award date. 2. 3. 4. E. Secondary vacancies may be awarded. The number of pilots awarded any BES may not differ by more than twenty percent (20%) or twenty (20) pilots, whichever is less, from the previous System Bid award unless previously agreed to by the SSC. Pilots may only be displaced if they are not staffed in their most recently awarded BES, or are not in the Hard Period for that BES. A pilot can only be displaced due to reductions in his awarded BES or by a more senior pilot that has displacement rights from another BES