It is possible to register language tags with IANA using the submission process described in RFC 3066. These tags can have 3- to 8-letter subtags in the second position.

While the i- prefix is reserved specifically for IANA tags, not all IANA tags begin with it. For example, a number of Chinese dialects have been registered with IANA. These include zh-guoyu, zh-hakka, zh-min, zh-min-nan, zh-wuu, etc.

Registering tags with IANA is better than using user-defined tags because it maximizes the likelihood of interoperability, due to the fact that the IANA tags are visible to others. On the other hand, IANA tags may be deprecated as new codes are added to the ISO standard. For this reason, there may be some risk to long-term interoperability when using certain IANA registered tags. This is particularly likely to apply to tags beginning with the i- prefix.

IANA tags that have been deprecated at the time this tutorial was published include no-bok (Norwegian "Book language" - use ISO 639 nb), i-navajo (Navajo - use ISO 639 nv), i-lux (Luxembourgish - use ISO 639 lb), and others.