period of service in grade, but not less than two years, for eligibility for consideration for promotion, in the case of officers designated for limited duty to whom paragraph (2) applies. (5) The Secretary of the Air Force may waive paragraph (2) to the extent necessary to assure that officers described in subparagraph (A) of such paragraph have at least two opportunities for consideration for promotion to the next higher grade as officers below the promotion zone. (6) In computing service in grade for purposes of this section, service in a grade held as a result of assignment to a position is counted as service in the grade in which the officer would have served except for such assignment or appointment. (b) Continued eligibility for consideration for promotion of officers who have previously failed of selection (1) Except as provided in paragraph (2), an officer who has failed of selection for promotion to the next higher grade remains eligible for consideration for promotion to that grade as long as the officer continues on active duty in other than a retired status and is not promoted. (2) Paragraph (1) does not apply to an officer on active status who is ineligible for consideration for promotion under section 631(c) of this title for the second time. (c) Officers To Be considered by promotion boards (1) Each time a selection board is convened under section 20211 of this title for consideration of officers in a competitive category for promotion to the next higher grade, each officer in the promotion zone (except as provided under paragraph (2)), and each officer above the promotion zone, for the grade and competitive category under consideration shall be considered for promotion. (2) The Secretary of the Air Force— (A) may, in accordance with standards and procedures prescribed by the Secretary of Defense in regulations which shall apply uniformly among the military departments, limit the officers to be considered by a selection board from below the promotion zone to those officers who are determined to