IETF language tags (conforming to the BCP 47 standard track and maintained in an IANA registry) are also partially derived from ISO 3166-1 alpha-2 codes (for the region subtags). The full list of ISO 3166-1 codes assigned to countries and territories are usable as region subtags. Also, the "exceptionally reserved" alpha-2 codes defined in ISO 3166-1 (with the exception of ) are also usable as region subtags for language tags. However, newer stability policies (agreed with ISO) have been implemented to avoid deleting subtags that have been withdrawn in ISO 3166-1; instead they are kept and aliased to the new preferred subtags, or kept as subtags grouping several countries. Some other region grouping subtags are derived from other standards. Under the newer stability policies, old assigned codes that have been withdrawn from ISO 3166-1 should no longer be reassigned to another country or territory (as has occurred in the past for "CS").