10 U.S.C. § 20238 — Establishment of promotion zones

(a) In General. Before a board considers officers above first lieutenant or lieutenant (junior grade) for promotion, the Secretary of the Air Force must set up a promotion zone for each grade and competitive category the board will consider. (b) Determination of Number. The Secretary decides how many eligible officers go in each zone, based on estimates of: (1) how many officers that category will need in the next higher grade over each of the next five years; (2) how many officers are projected to actually be serving in that higher grade each of those years; (3) for grades covered by section 523, how many officers on sustained duty are authorized in that grade at the end of each of the next five fiscal years; and (4) how many officers should be placed in the zone each year so officers get roughly similar promotion chances year to year.