10 U.S.C. § 10145 — Ready Reserve: placement in

(a) Each person required under law to serve in a reserve component must, upon becoming a member, be placed in the Ready Reserve of his armed force for his prescribed term of service, unless he is transferred to the Standby Reserve under section 10146(a) of this title.
(b) The units and members of the Army National Guard of the United States^* and of the Air National Guard of the United States^* are in the Ready Reserve of the Army^* and the Ready Reserve of the Air Force, respectively.
(c) All Reserves assigned to units organized to serve as units and designated as units in the Ready Reserve are in the Ready Reserve.
(d) Under such regulations as the responsible Secretary can prescribe, any qualified member of a reserve component or any qualified retired enlisted member^* of a regular component can, upon his request, be placed in the Ready Reserve. However, a member of the Retired Reserve entitled to retired pay^* or a retired enlisted member of a regular component cannot be placed in the Ready Reserve unless the responsible Secretary makes a special finding that the member’s services in the Ready Reserve are indispensable. The authority of the responsible Secretary under the preceding sentence cannot be delegated—
(1) to a civilian officer^* or employee of the military department concerned below the level of Assistant Secretary; or
(2) to a member of the armed forces^* below the level of the lieutenant general or vice admiral in an armed force with responsibility for military personnel policy in that armed force.