Relation To Other Standards
Although subtags are often derived from ISO standards, they do not follow these standards absolutely, as this could lead to the meaning of language tags changing over time.
In particular, a subtag derived from a code assigned by ISO 639, ISO 15924, ISO 3166 (or UN M.49 only for supranational geographical regions) remains a valid (though deprecated) subtag even if the code is withdrawn from the corresponding ISO standard. If the ISO standard later assigns a new meaning to the withdrawn code, the corresponding subtag will still retain its old meaning.
This stability was introduced in the (now obsolete) RFC 4646 (and confirmed in its successor). Before RFC 4646, changes in the meaning of ISO codes could cause changes in the meaning of language tags.
Read more about this topic: IETF Language Tag
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