A Starfleet order whose sections B and C govern how a senior officer may take command of a starship and how he may be relieved, both invoked in The Doomsday Machine.
Starfleet Order 104 deals with the transfer of command aboard a starship. Section B allows an officer of flag rank to take command of a vessel, and Section C allows such an officer to be relieved if he is shown to be medically or psychologically unfit.
Both sections are used in The Doomsday Machine. Commodore Matt Decker, the traumatised sole survivor of the USS Constellation, cites Section B to take the Enterprise from Spock and throws the ship into a hopeless attack on the planet killer. Spock points to Section C, but McCoy has had no chance to examine Decker and cannot certify him unfit.
The deadlock ends only when Kirk, speaking from the wrecked Constellation, orders Spock to take command on his personal authority, and Spock makes clear that he will place Decker under arrest if he resists.