[2] It provides an enumeration of languages as complete as possible, including living and extinct, ancient and constructed, major and minor, written and unwritten.
In archives and other information storage, it is used in cataloging systems, indicating what language a resource is in or about.
The codes are also frequently used in the linguistic literature and elsewhere to compensate for the fact that language names may be obscure or ambiguous.
Since the code is three-letter alphabetic, one upper bound for the number of languages that can be represented is 26 × 26 × 26 = 17,576.
According to section 6.4.1 of the standard, they may only contain the 26 letters the ISO basic Latin alphabet and diacritics from the following set: Reference names may also include any of five ASCII punctuation marks [space, hyphen-minus, apostrophe and the parentheses], and within parentheses the full stop and the ten digits.
Others like nor (Norwegian) had their two individual parts (nno (Nynorsk), nob (Bokmål)) already in ISO 639-2.
This is an attempt to deal with varieties that may be linguistically distinct from each other, but are treated by their speakers as two forms of the same language, e.g. in cases of diglossia.
These are intended primarily for applications like databases where an ISO code is required regardless of whether one exists.
For example, constructed languages can be encoded, but only if they are designed for human communication and have a body of literature, preventing requests for idiosyncratic inventions.
The registration authority documents on its Web site instructions made in the text of the ISO 639-3 standard regarding how the code tables are to be maintained.
Based on consensus in comments received, a change request may be withdrawn or promoted to "candidate status".
Three months prior to the end of an annual review cycle (typically in September), an announcement is sent to the LINGUIST discussion list and other lists regarding Candidate Status Change Requests.
Decisions are announced at the end of the annual review cycle (typically in January).
At that time, requests may be adopted in whole or in part, amended and carried forward into the next review cycle, or rejected.
But he raises doubts about industry need for the comprehensive coverage provided by ISO 639-3, including as it does "little-known languages of small communities that are never or hardly used in writing and that are often in danger of extinction".