 The second option is called "extlang form" and is new in RFC 5646.Whole tags that were registered prior to RFC 4646 and are now classified as "grandfathered" or "redundant" (depending on whether they fit the new syntax) are deprecated in favor of the corresponding ISO 639-3–based language subtag, if one exists.To list a few examples, "nan" is preferred over "zh-min-nan" for Min Nan Chinese; "hak" is preferred over "i-hak" and "zh-hakka" for Hakka Chinese; and "ase" is preferred over "sgn-US" for American Sign Language.