Unicode locale extension BCP 47 Extension U published as RFC 6067
RFC 6067 specifies an extension to BCP 47. BCP 47 provides subtags that specify language and/or locale-based behavior.
Many locale identifiers require additional “tailorings” or options for specific values within a language, culture, region, or other variation. This extension provides a mechanism for using these additional tailorings within language tags for general interchange.
The maintaining authority for the extension defined by this document is the Unicode Consortium. The Unicode Consortium defines a standardized, structured set of locale data and identifiers for locale data in the “Common Locale Data Repository” or “CLDR”.