10 U.S.C. § 12320

A reserve officer who is ordered to active duty or full-time National Guard duty shall be ordered to active duty or full-time National Guard duty in his reserve grade, except that a reserve officer who is credited with service under section 12207 of this title and is ordered to active duty and placed on the active-duty list may be ordered to active duty in a reserve grade and with a date of rank and position on the active-duty list determined under regulations prescribed by the Secretary of Defense based upon the amount of service credited.