Halliday & Matthiessen (1999: 172):
The different degrees of involvement are reflected in the way the figure, and its elements, are realised in the grammar of the clause. A participant is realised as a nominal group, and is typically placed next before or next after the verbal group realising the process. Circumstances typically occur further away from the process, and are of two distinct types. Type 1, simple circumstance, represents a quality; this type is realised by an adverbial group. Type 2, macro circumstance, is realised as a prepositional phrase, which in turn consists of a preposition + nominal group.