Halliday & Matthiessen (2004: 592-3):
… there are realignments in the realisational relationship between semantic units and grammatical ones. … a sequence is realised by a clause complex; the combination of a figure, a proposition (proposal), and a message is realised by a clause; … a participant is realised by a nominal group, a process by a verbal group and a circumstance by and adverbial group or a prepositional phrase. But once these couplings between the two strata of the content plane have been established, “cross–couplings” become theoretically possible. For example, while sequences are realised by clause complexes and figures by clauses, it is theoretically possible that, under certain conditions, sequences would be realised by clauses — that is, as if they were figures. This is the possibility of metaphorical realisation … . But the two forms of realisation are not, of course, synonymous, so the effect is one of expanding the meaning potential of the language.