ISO 21636 provides a framework for the identification and description of varieties of all individual human languages, including sign languages (cf. ISO 639). This excludes artificial means of communication with or between machines (such as programming languages) and those means of human communication which are not fully nor largely equivalent to human language (such as sets of individual symbols or gestures that each carry isolated meanings but cannot be freely combined into complex expressions). This document, Part 3 of ISO 21636, provides general instructions on how to apply the framework to identify basic dimensions and sub-dimensions of linguistic variation and the resulting varieties, including major modalities of human communication. It does not include any code nor individual identifiers.