10 U.S.C. § 12306

(a) Units and members in the Standby Reserve may be ordered to active duty only as provided in section 12301 of this title , but subject to the limitations in subsection (b). (b) In time of emergency— (1) no unit in the Standby Reserve organized to serve as a unit or any member thereof may be ordered to active duty under section 12301(a) of this title , unless the Secretary concerned, with the approval of the Secretary of Defense in the case of a Secretary of a military department, determines that there are not enough of the required kinds of units in the Ready Reserve that are readily available; and (2) notwithstanding section 12301(a) of this title , no other member in the Standby Reserve may be ordered to active duty as an individual under such section without his consent, unless the Secretary concerned, with the approval of the Secretary of Defense in the case of a Secretary of a military department, determines that there are not enough qualified members in the Ready Reserve in the required category who are readily available.