Halliday & Matthiessen (2014: 571):
At the deepest level time-phase and reality-phase are the same thing: both are concerned with the stages of becoming. A process is something that emerges out of imagination into reality, like the rising of the sun. Before dawn, the sun shines only in the future, or only in the imagination — as future turns into present, imagination turns into reality. 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.