Halliday & Matthiessen (1999: 128):
There are two interlocking aspects of the configuration of a figure:
(i) the domain of experience to which the figure belongs; and
(ii) the nature of the interaction among its participants.
To put this another way: as a theory of experience, the semantic system of figures embodies two subtheories: one concerning different domains of experience and one concerning the ways in which participating phenomena can interact.