Halliday & Matthiessen (1999: 526):
The latter [derived from the polarity of proposals] occupy the dimensions of "obligation" and "readiness", readiness including both inclination and ability; for example,They ought to clean this place up. People will leave it so untidy.— They can't; they haven't got the equipment. They're not supposed to clean it anyway.— You mean they won't. But someone must. Can we?— I don't see whey we shouldn't. Will you help?Although these are derived from the sense of proposals ('you are required/supposed/allowed; I am able/willing'), they are not restricted to clauses having these speech functions; obligation and readiness are construed by the grammar propositionally and hence are used freely with third persons. But they still represent the judgments of speaker or listener on the obligations or inclinations involved (he ought to help, she will help).