Halliday & Matthiessen (2004: 618):
If the clause is a ‘goods–&–services’ clause (a proposal, which has no real congruent form in the grammar, but by default we can characterise it as imperative), it means either (i) ‘is wanted to’, related to a command, or (ii) ‘wants to’, related to an offer; in other words, some degree of obligation [‘deontic’ modality] or inclination. … Note that modulation refers to the semantic category of proposals; but all modalities are realised as indicative (that is, as if they were propositions).