Originally Posted by ATL7ER

Every pilot who moves position on an AE creates a vacancy. That vacancy will either be filled or not based on how many numbers the company wants in that category/seat after this AE. That's why, if category numbers are remaining constant, pilots get awarded positions that were not posted as vacancies on the AE posting...i.e. a NYC 7ER F/O takes one of the ATL 717A positions, someone else, say a M88 FO, gets awarded NYC 7ER FO even though the AE showed no NYC7ER FO vacancies. If they wanted the NYC7ERFO category to shrink by 1 position they wouldn't "backfill" that guys slot and the M88 FO would not get that award.

Keep in mind also that the numbers they talk about in the AE posting are subject to change by the time they actually make awards.