 The numbers following these prefixes can be "special" four-digit numbers, or domestic two- or three-digit numbers or simply the domestic North American "RETMA" numbering system.Like the British military system, these have many direct equivalents in the civilian types.Confusingly, the British also had two entirely different "VT" nomenclatures, one used by the Royal Air Force (see the preceding section) and the other used by the General Post Office, responsible for post and telecommunications at the time, where it may have stood for "valve, telephone"; none of these schemes corresponded in any way with each other.