ISO 4217 "ruble". In addition to codes for most active national currencies ISO 4217 provides codes for "supranational" currencies, procedural purposes, and several things which are "similar to" currencies: The use of an initial letter "X" for these purposes is facilitated by the ISO 3166 rule that no official country code beginning with X will ever be assigned. Because of this rule ISO 4217 can use X codes without risk of clashing with a future country code. ISO 3166 country codes beginning with "X" are used for private custom use (reserved), never for official codes. For instance, the ISO 3166-based NATO country
ISO/TC 37 are closely linked to the standardization efforts of IEC (International Electrotechnical Commission, founded in 1906) and ISO (International Organization for Standardization, founded in 1946). A terminology standard according to ISO/IEC Guide 2 (1996) is defined as "standard that is concerned with terms, usually accompanied by their definitions, and sometimes by explanatory notes, illustrations, examples, etc." ISO 1087-1:2000 defines terminology as ""set of designations belonging to one special language" and designations as "representation of a concept by a sign which denotes it"." Here, concept representation goes beyond terms (being only linguistic signs), which is also supported by the state-of-the-art of terminology