The reserve activation clause was a Starfleet regulation which was in force during the 2270s. Described as "little-known" and "seldom-used," the clause provided for the recall of a discharged or retired Starfleet officer to active duty in response to a crisis or emergency.
In the mid-2270s, Fleet Admiral Nogura, at the request of Rear Admiral James T. Kirk, recalled Leonard McCoy to active duty in the Starfleet using the reserve activation clause. Upon reporting to the USS Enterprise, Doctor McCoy stated "In simpler language... they drafted me!" (Star Trek: The Motion Picture)