The Sub-Coordinator was the second-in-command, behind the Coordinator, of the Celestial Intervention Agency.
Narvin was the Sub-Coordinator towards the end of the Fourth Doctor's life. (AUDIO: Erasure) He later succeeded Vansell as Coordinator. (AUDIO: Weapon of Choice) Later, when Romana II installed herself as Coordinator of the CIA after stepping down from the position of Lady President of Gallifrey, Narvin was returned to the position of second-in-command of the organisation. In that instance, however, he took the title of Deputy Coordinator. (AUDIO: Enemy Lines)