On RFC 2119

RFC 2119 standardizes the meanings of requirement keywords such as MUST, SHOULD and MAY. These definitions only apply if the keywords appear in uppercase, as clarified 20 years later in RFC 8174.

One thing to note is that some native German speakers might misinterpret MUST NOT as do not have to. In affirmative statements English must aligns with German müssen, but the correct back-translation of müssen is have to, whose negation connotes lack of obligation rather than prohibition. English must does not have a single German equivalent. Under positive polarity it corresponds to müssen but under negative polarity it corresponds to the negation of dürfen (i.e. the negation of may). The same split appears in other languages as well, but it’s particularly salient in German due to must and müssen (and it’s forms muss and musst) sounding so similar despite being modal opposites under negation.

On April 1st you could—but probably shouldn’t—also use keywords defined in RFC 6919.