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
International Classification for Standards International Classification for Standards International Classification for Standards (ICS) is an international classification system for technical standards. It is designed to cover every economic sector and virtually every activity of the humankind where technical standards may be used. Developed and maintained by the International Organization for Standardization, the ICS is intended to be a continuous work in progress and is updated when necessary. The latest edition of the ICS can be downloaded free of charge from the ISO web site. Anyone may submit a proposal for modifications or additions to the ICS. The ICS serves as a structure for catalogues