Celatron was one of many planets conquered by the Dalek Empire during their invasion of Mutter's Spiral. A Red Dalek was Supreme Controller of a sector of this planet. (AUDIO: "Death to the Daleks!")
Human historians in the post-Time War universe, following the Siege of Trenzalore, acknowledged Selatron as a world which had fallen to the expanding Dalek Empire. (PROSE: Dalek: The Astounding Untold History of the Greatest Enemies of the Universe)