 The Planes are numbered according to their order away from the Prime Plane - which is a term used by human wizards for their own plane.As noted by Zdim in the first paragraph of "The Fallible Fiend", there is nothing objective about this designation - any Plane could term itself "Prime" and number the others accordingly.Some of the Planes get detailed descriptions - in particular, Zdim's Twelfth Plane - while others get only a brief mention.