Halliday & Matthiessen (1999: 52-3):
A figure is a representation of experience in the form of a configuration, consisting of a process, participants taking pan in this process and associated circumstances. There are, of course, indefinitely many kinds of process in the non-semiotic world; but these are construed semiotically, according to the way in which they configure participants, into a small number of process types — being, doing, sensing, and saying. The first three of these have clearly defined subcategories: see Plate 3.
Then, each figure may be either projected (by another figure, or in some other way) or else not; and if projected, it may be an idea or locution or [fact] …