Halliday & Matthiessen (1999: 57-8):
As already mentioned, there is a typology of figures based largely on the nature of the particular types of the process, participant, and circumstance roles; the most general part of this is shown in Figure 2-6.
Each type of figure has its own set of more delicately specified roles with particular value restrictions. For example, figures of doing have an Actor, which in turn is a participant (rather than some other kind of phenomenon). (Notice that the specification of the filler of the Phenomenon role in a sensing figure is more general than 'participant' — any type of phenomenon can be sensed.)