Source: Dear AFA (September 4, 2012)

The first step in the annual vacation bidding process is the assignment of Reserve letters. 

Section 10.A. of our Contract requires that 2013 Reserve letters be assigned and posted in domiciles in September which is the month prior to the start of vacation bidding. The list establishes the top 25% of the domicile who are exempt from Reserve rotation and assigns "A" or "B" to all Flight Attendants with more than five years seniority. 

Reserve letter lists, otherwise known as A/B Reserve Rotation Letter Lists, are scheduled to be available in the domicile around September 29, 2012. These Reserve letters become effective for the February 2013 schedule month.