ISO 639-6:2009 specifies a method for establishing four-letter language identifiers (alpha-4) and language reference names for language variants and a hierarchical framework for relating them to languages, language families and language groups. The alpha-4 language identifiers have been developed for use in a wide range of applications, especially in computer systems, where there is a potential need to cover the entire range of languages, language families and language groups as well as language variants within each identified language. Alpha-4 language identifiers can support the quantity of known language variants and accommodate any future expansion.

ISO 639-6:2009 provides a hierarchical framework, which facilitates backward compatibility with other ISO 639 codes, based on linguistic and/or geolinguistic relationships, within which a comprehensive enumeration of language variants is possible, including living, extinct, ancient and constructed languages, whether major or minor. As a result, ISO 639-6:2009 caters for a very large number of languages and their variants. ISO 639-6:2009 is not applicable to the registrations for languages designed exclusively for machine use, such as computer-programming languages and reconstructed languages.