Halliday & Matthiessen (1999: 172):
As we have already noted, a figure consists of a process, participants involved in the process, and associated or attendant circumstances. Of these, the process can be seen as the organisational centre — the element that reflects the relative arrangements of the other parts in the configuration. These other parts (participants and circumstances) are more or less closely involved in the actualisation of the process. Broadly speaking, participants are directly involved in the process; circumstances are more peripherally attendant on it.