Halliday & Matthiessen (1999: 53-4):
The principle of organisation of a figure is different from that of a sequence. As we have seen, a sequence is constructed by interdependency relations of expansion and projection. In contrast, a figure is constructed as an organic configuration of parts. Each part stands in a specific relation to the figure as a whole. The parts of a given configuration are (i) a nuclear process, (ii) one to three participants of different kinds taking part in the process, and (iii) up to around seven circumstances of different kinds associated with it.
Participants are inherent in the process; they bring about its occurrence or mediate it. There are a number of specific ways in which a participant may take part in a process; it may act out the process, it may sense it, it may receive it, it may be affected by it, it may say it, and so on. The different configurations of participants are the bases for a typology of process types. The distinction between participants and circumstances is a cline rather than a sharp division, but it is semantically quite significant.
Circumstances are typically less closely associated with the process and are usually not inherent in it. They specify the spatial or temporal location of the process, its extent in space or time (distance or duration), its cause, the manner of its occurrence, and so on.