10 USC 620 - Active-duty lists

(a) The Secretary of the military department concerned shall maintain a single list of all officers (other than officers described in section 641 of this title) who are on active duty for each armed force under his jurisdiction (other than the Coast Guard when it is operating as a service in the Navy).
(b) Officers shall be carried on the active-duty list of the armed force of which they are members in the order of seniority of the grade in which they are serving on active duty. Officers serving in the same grade shall be carried in the order of their rank in that grade.
(c) An officer whose position on the active-duty list results from service under a temporary appointment or in a grade held by reason of assignment to a position has, when that appointment or assignment ends, the grade and position on the active-duty list that he would have held if he had not received that appointment or assignment.
(d) Under regulations prescribed by the Secretary of the military department concerned, a reserve officer who is ordered to active duty (whether voluntarily or involuntarily) during a war or national emergency and who would otherwise be placed on the active-duty list may be excluded from that list as determined by the Secretary concerned. Exclusion of an officer from the active-duty list as the result of action by the Secretary concerned under the preceding sentence shall expire not later than 24 months after the date on which the officer enters active duty under an order to active duty covered by that sentence.