10 U.S.C. § 12643

(a) Except as provided in section 612(a)(3) of this title and except for boards that may be convened to select Reserves for appointment in the Regular Army, Regular Navy, Regular Air Force, or Regular Marine Corps, each board convened for the appointment, promotion, demotion, involuntary release from active duty, discharge, or retirement of Reserves shall include at least one member of the Reserves, with the exact number of Reserves determined by the Secretary concerned in his discretion. (b) Each member of a board convened for the selection for promotion, or for the demotion or discharge, of Reserves must be senior in rank to the persons under consideration by that board. However, a member serving in a legal advisory capacity may be junior in rank to any person, other than a judge advocate or law specialist, being considered by that board; and a member serving in a medical advisory capacity may be junior in rank to any person, other than a medical officer, being considered by that board.