; and (B) the estimated number of officers on sustained duty and not on sustained duty needed to fill vacancies in those positions over the 24-month period beginning on the date on which the selection board convenes. 20238. Establishment of promotion zones (a) In general Before convening a selection board under section 20211 of this title to consider officers for promotion to any grade above first lieutenant or lieutenant (junior grade), the Secretary of the Air Force shall establish a promotion zone for officers serving in each grade and competitive category to be considered by the board. (b) Determination of number The Secretary of the Air Force 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. Such determination shall be made on the basis of an estimate of— (1) the number of officers needed in that competitive category in the next higher grade in each of the next five years; (2) the number of officers to be serving in that competitive category in the next higher grade in each of the next five years; (3) in the case of a promotion zone for officers to be promoted to a grade to which section 523 of this title is applicable, the number of officers authorized for such grade under such section to be on sustained duty on the last day of each of the next five fiscal years