Halliday & Matthiessen (1999: 178):
Being elements, participants and processes occupy rôles in figures; but whereas processes only serve in the the single rôle of Process, participants range over a much wider experiential spectrum — the direct participant rôles of Actor, Goal, Senser, Phenomenon, and so on, and also the indirect participant rôles within circumstances such as Location and Cause. Thus, seen from the point of view of figures, participants are construed as being experientially more complex, in the sense that they can take a variety of configurational rôles.