Halliday & Matthiessen (1999: 64-5):
Processes serve in the most central or nuclear rôle in a figure; they embody the temporal properties of a figure unfolding in time: see Plate 9. Other than metaphorical processes, the process element is either polar (positive/negative) or modal (some intermediate degree between positive and negative); it may embody phase, or aspect; and it will refer to past, present or future time. Polarity and modality derive from the interpersonal perspective on the process.
