A category nine destruction rig was a type of military vessel used by Wrechon Four. It was not only a ship, but a bomb as well, capable of destroying half a planet.
Commander Kohrbal and Pheenan of the genocide squad travelled to Teymah to destroy the Ninexie, the Commander activating the self-destruct upon his death. Using telepathy, the Sixth Doctor was able to send Constance Clarke the access code, Gamma, Gamma, Three, Delta, Nine, and cancelled the destruct sequence. (AUDIO: Absolute Power)