Halliday & Matthiessen (1999: 558):
There is a great deal of indeterminacy throughout the region; but it is of more than one kind. At one "corner", if we combine value with neutral (immediate), then the resulting wordings are ambiguous, as to the type of modality expressed: thus must has three clearly distinct meanings, (a) as probability (e.g. that must be Mary 'certainly that is Mary'), (b) as obligation (e.g. you must wear a helmet 'it is essential that you wear a helmet'), (c) as readiness: inclination (e.g. if you must make all that noise 'if you insist on making all that noise’). That these are truly ambiguous can be gathered from an example such as she must complain, which has to be interpreted in one or another of these different meanings — the context will of course usually make it clear which;