((a)) Before convening a selection board to recommend officers for promotion to any grade above lieutenant (junior grade) and below rear admiral (lower half), the Secretary shall establish a promotion zone for the grade to be considered. The promotion zone for each grade shall consist of the most senior officers of that grade on the active duty promotion list who are eligible for consideration for promotion to the next higher grade and who have not previously been placed in a promotion zone for selection for promotion to the next higher grade. The number of officers in each zone shall be determined after considering—

((1)) the needs of the service;

((2)) the estimated numbers of vacancies available in future years to provide comparable opportunity for promotion of officers in successive year groups; and

((3)) the extent to which current terms of service in that grade conform to a desirable career promotion pattern.

((b)) Promotion zones from which officers will be selected for promotion to the grade of rear admiral (lower half) shall be established by the Secretary as the needs of the service require.