The command packet was a small pouch kept in the posession of Federation starship captains, containing the ship's itinerary and destination schedule. As this was considered to be sensitive information, Starfleet captains were usually the only individuals with access to the packet. Captain Kirk kept it in his personal safe in his quarters.
In 2266, an android posing as Captain Kirk retrieved the packet from Kirk's safe and delivered it to Roger Korby, who reviewed the Enterprise's destination schedule to determine the best location to start his covert infiltration of society with androids. (TOS: "What Are Little Girls Made Of?")
Captain Kirk also kept a record tape containing his final orders to Mister Spock and Doctor McCoy in his command packet, should he ever be declared dead. In 2268, when it appeared as if Kirk had been lost aboard the Starship Defiant, Spock and McCoy reviewed the tape together. (TOS: "The Tholian Web")