10 U.S.C. § 10141

(a) There are in each armed force a Ready Reserve, a Standby Reserve, and a Retired Reserve. Each Reserve shall be placed in one of those categories. (b) Reserves who are on the inactive status list of a reserve component, or who are assigned to the inactive Army National Guard or the inactive Air National Guard, are in an inactive status. Members in the Retired Reserve are in a retired status. All other Reserves are in an active status. (c) As prescribed by the Secretary concerned, each reserve component except the Army National Guard of the United States and the Air National Guard of the United States shall be divided into training categories according to the degrees of training, including the number and duration of drills or equivalent duties to be completed in stated periods. The designation of training categories shall be the same for all armed forces and the same within the Ready Reserve and the Standby Reserve.