Modalisation

Halliday & Matthiessen (2004: 618):

If the clause is an ‘information’ clause (a proposition, congruently realised as indicative), this means either (i) ‘either yes or no’, that is, ‘maybe’; or (ii) ‘both yes and no’, that is, ‘sometimes’; in other words, some degree of probability [‘epistemic’ modality] or usuality.