10 U.S.C. § 1187

(a) In General .— Except as provided in subsection (b), each board convened under this chapter shall consist of officers appointed as follows: (1) Each member of the board shall be an officer of the same armed force as the officer being required to show cause for retention on active duty. (2) Each member of the board shall be in a grade above major or lieutenant commander, except that at least one member of the board shall be in a grade above lieutenant colonel or commander. (3) Each member of the board shall be senior in grade to any officer to be considered by the board. (b) Retired Officers .— If qualified officers are not available in sufficient numbers to comprise a board convened under this chapter, the Secretary of the military department concerned shall complete the membership of the board by appointing to the board retired officers of the same armed force. A retired officer may be appointed to such a board only if the retired grade of that officer— (1) is above major or lieutenant commander or, in the case of an officer to be the senior officer of the board, above lieutenant colonel or commander; and (2) is senior to the grade of any officer to be considered by the board. (c) Ineligibility by Reason of Previous Consideration of Same Officer .— No person may be a member of more than one board convened under this chapter to consider the same officer. (d) Exclusion From Strength Limitation .— A retired general or flag officer who is on active duty for the purpose of serving on a board convened under this chapter shall not, while so serving, be counted against any limitation on the number of general and flag officers who may be on active duty.