The reserve activation clause was a Starfleet regulation which was in force during the 2250s and 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 2250s, during a simulated Tholian attack designed to test Cadet Thira Sidhu's decision-making abilities in a crisis, Capt. Christopher Pike (who had supposedly been relieved of duty) suggested that Cadet Sidhu use the reserve activation clause to restore him to duty. Cadet Sidhu quoted the clause as: "An officer in good standing may reinstate a discharged officer in an emergency," and called it "a loophole." (ST: "Ask Not")
In the mid-2270s, Fleet Admiral Nogura, at the request of Rear Admiral James T. Kirk, recalled Dr. Leonard McCoy to active duty in the Starfleet using the reserve activation clause. Upon reporting to the USS Enterprise, McCoy explained his recall thusly to Kirk: "In simpler language, captain, they drafted me!" (Star Trek: The Motion Picture)