Halliday & Matthiessen (1999: 178):
However, there is a significant difference between elements and figures in the nature of their internal organisation. While figures consist of phenomena that are ontologically of different types — participants, processes and circumstances, the components of an element belong in principle to the same type. That is, the components of a participant are themselves potential participants, and the components of a process are themselves potential processes. Grammatically speaking, participants are realised by nominal groups, which are groups of nouns; and processes are realised by verbal groups, which are groups of verbs. The limiting case of a group is a single word.