Other parts of ISO 639 have included identifiers designated as individual language identifiers that correspond in a one-to-many manner with individual language identifiers in this part of ISO 639. For instance, ISO 639-3 contains over 30 identifiers designated as individual language identifiers for distinct varieties of Arabic, while ISO 639-1 and ISO 639-2 each contain only one identifier for Arabic, "ar" and "ara" respectively, which are designated as individual language identifiers in those parts of ISO 639. It is assumed here that the single identifiers for Arabic in parts 1 and 2 of ISO 639 correspond to the many identifiers collectively for distinct varieties of Arabic in part 3 of ISO 639.
In this example, it may appear that the single identifiers in ISO 639-1 and ISO 639-2 should be designated as collective language identifiers. That is not assumed here, however. In various parts of the world, there are clusters of closely-related language varieties that, based on the criteria discussed in 4.2.1, can be considered individual languages, yet in certain usage contexts a single language identity for all is needed. Typical situations in which this need can occur include the following:
- There is one variety that is more developed and that tends to be used for wider communication by speakers of various closely-related languages; as a result, there is a perceived common linguistic identity across these languages. For instance, there are several distinct spoken Arabic languages, but Standard Arabic is generally used in business and media across all of these communities, and is also an important aspect of a shared ethno-religious unity. As a result, a perceived common linguistic identity exists.
- There is a common written form used for multiple closely-related languages. For instance, multiple Chinese languages share a common written form.
- There is a transitional socio-linguistic situation in which sub-communities of a single language community are diverging, creating a need for some purposes to recognize distinct languages while, for other purposes, a single common identity is still valid. For instance, in some business contexts it is necessary to make a distinction between Bosnian, Croatian and Serbian languages, yet there are other contexts in which these distinctions are not discernible in language resources that are in use.
Where such situations exist, an identifier for the single, common language identity is considered in this part of ISO 639 to be a macrolanguage identifier.
Macrolanguages are distinguished from language collectives in that the individual languages that correspond to a macrolanguage must be very closely related, and there must be some domain in which only a single language identity is recognized.