A lot of commentary here on if "MUST NOT" is too ambiguous, or not, or a problem for non-native speakers...

This makes the authors point about citation. In technical documentation any terms use like this must (MUST) be defined, either in line or by citation.

This is also part of the reason they are capitalized, to key you to the fact they are being used in a structurally important way. If you are reading them, you need to familiarize yourself with the context.