Halliday & Matthiessen (2004: 501):
The two categories of phase are related to modality and tense; but whereas modality and tense are interpreted as subcategorisations of one process (they are grammatical variants within one verbal group), phase is interpreted as a hypotactic relation between two processes: a general one of becoming, that is then elaborated by the specific action, event, mental process or relation that is being phased in or out.