Halliday & Matthiessen (2014: 692):
Note that modulation refers to the semantic category of proposals; but all modalities are realised as indicative (that is, as if they were propositions). Thus imperative go home!, when modulated, becomes indicative you must go home! (In philosophical semantics and linguistic accounts influenced by ‘modal logic’, probability is referred to as ‘epistemic’ modality (from Greek episteme, for ‘knowledge’) and obligation as ‘deontic’ modality (from Greek deon, for ‘what is binding’). Readiness has been referred to as ‘dynamic’ modality, and deontic and dynamic modalities have been grouped together as ‘root’ modality – our modulation type of modality; but usuality tends to be left out of accounts of modality.