Halliday & Matthiessen (1999: 550):
(1) Ambiguity(i) must:You must be very careful! (when you do that)
— obligation 'it is essential that you should be'
You must be very careless! (to have done that)
— probability 'I am certain that you were'
Here the listener/reader adopts either one interpretation or the other — usually, of course, without noticing that there is another possible meaning. In (i), the Attribute suggests the choice (one does not usually instruct someone to be careless!); but cf. you must be very sure of yourself ('before you do that'/ 'to have done that'). Compare also (ii), where the ambiguity is one that is typical of identifying clauses.(ii) home is where your heart is
— Token ^ Value 'if you live in a place, you love it'— Value ^ Token 'if you love a place, it is home to you'