True enough, JTT. "Must" does carry authority. What I don't understand is, "External authority or speaker's authority." Anybody, regardless of rank or position, can use "must." "Have to" is less formal, but the CEO of a corporation would not be inappropriate in saying, "They have to do this."
What am I missing?
Rules, laws, regulations, etc are all examples of external authority, Roberta but I'm sure you know that already. But just because anybody can use 'must', it is part of our language after all, it's the situation/ the context that it's used in that makes all the difference.
We can use 'may' for permission but for most situations we don't; native speakers opt for 'can' or 'could'. We can all use 'shall' but 'shall' used in laws/regulations connotes something slightly different. In language we're stuck with a limited number of modals to express an infinite number of thoughts/idea but luckily these modals can morph depending on the situation, even upon a speaker's intonation.
I'm not saying that there is anything major to this distinction, but it is out there. Perhaps Navigator could tell us where she/he found it.
My feeling is, and this is only my feeling, that this is another prescription that, like many of them, got out of control; a poor analysis of language was done.
I fully agree with you that any CEO could use 'have to' as you've noted. What might happen if that CEO set down a whole list of "have to's" in a meeting and an underling put them into a formal company memo as a new set of rules?