ISO 639-6:2009规定了为语言变体建立四字母语言标识符（alpha-4）和语言参考名称的方法，以及将其与语言、语族和语言组关联的层次结构。alpha-4语言标识符已被开发用于广泛的应用，尤其是在计算机系统中，其中可能需要涵盖所有语言、语系和语组，以及每种已识别语言中的语言变体。Alpha-4语言标识符可以支持已知语言变体的数量，并适应未来的任何扩展。 ISO 639-6:2009提供了一个层次结构框架，该框架基于语言和/或地理语言关系，促进了与其他ISO 639代码的向后兼容性，在该框架内，可以全面列举语言变体，包括活的、灭绝的、古老的和构造的语言，无论是主要的还是次要的。 因此，ISO 639-6:2009适用于大量语言及其变体。ISO 639-6:2009不适用于专门为机器使用而设计的语言的注册，例如计算机编程语言和重构语言。

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.