The same way that a dictionary definition clarifies the meaning of a single English word. By using complete sentences to rule out grey areas.
I see no point in furthering this discussion. Your defeatist assertions like the above "there's no possible way to disambiguate English", your refusal to admit that apparent synonyms such as "must" and "shall" might not be perfect synonyms to people other than yourself, and your repeated sarcasm and snark, suggest to me that you do not see the opposite viewpoint as legitimate, and therefore are not amenable to changing your viewpoint.
Right, so use the dictionary we already have! Why do we need the RFC?
> "there's no possible way to disambiguate English"
This quote isn't from me. I didn't say that - I don't know where you've got it from.
> your repeated sarcasm and snark
Sarcasm and snark? I don't use those rhetorical devices if I can avoid it. I don't think I've used them in this thread at all! I don't know where you've got the idea that I'm not arguing seriously from. I'm sorry I gave that impression.
My legitimate argument is that nobody is honestly confused by words like must - nobody. The RFC is over-the-top formalism where it's not needed.
To me it seems that there could easily be confusion between these two interpretations.
These words already have definitions. You can look them up in a dictionary. Or almost all speakers know them anyway.
Why do we need an RFC, and to write them in capital letters?
What does the RFC add to anything? If you removed the capitalisation and the reference to the RFC then your text means exactly the same thing, as the words already had the same meanings.
> English is ambiguous
Well then how does the RFC solve that problem? Their definitions of these terms are using other English words which are also ambiguous.
> I for one, can never remember whether SHALL is mandatory or not
What do you think 'shall' means in a normal conversation? How could it mean anything except mandatory? In what normal English context does 'shall' mean optional?
If a normal person tells you that you must submit your expenses before the end of the month do you get confused about whether it's required or not? If they write it in capitals and refer you to the RFC do you suddenly understand now?