10 U.S.C. § 14305

(a) Establishment of Zone .— Before convening a mandatory promotion board under section 14101(a) of this title , the Secretary of the military department concerned shall establish a promotion zone for officers serving in each grade and competitive category to be considered by the board. (b) Number in the Zone .— The Secretary concerned shall determine the number of officers in the promotion zone for officers serving in any grade and competitive category from among officers who are eligible for promotion in that grade and competitive category under the provisions of sections 14303 and 14304 of this title and who are otherwise eligible for promotion. (c) Factors in Determining Number in the Zone .— The Secretary’s determination under subsection (b) shall be made on the basis of an estimate of the following: (1) The number of officers needed in that competitive category in the next higher grade in each of the next five years. (2) In the case of a promotion zone for officers to be promoted to a grade to which the maximum years of in grade criteria established in section 14304 of this title apply, the number of officers in that competitive category who are required to be considered for selection for promotion to the next higher grade under that section. (3) The number of officers that should be placed in the promotion zone in each of the next five years to provide to officers in those years relatively similar opportunities for promotion.