Halliday & Matthiessen (1999: 265):
Participants are realised by nominal groups, which allow more or less indefinite expansion (through the univariate structure of modification). This expansion is the grammar’s way of constructing taxonomies of things: grouping them into classes, assigning properties to them, quantifying them and then uniquely identifying any individual thing, or any number, set or class of things in relation to the “here–&–now” of the speech event. the expansion involves open sets of things and qualities, realised by lexical items; but it can also capture a circumstance, realised by a prepositional phrase, or an entire figure, realised by a clause, and put it to use as a quality in describing or identifying such a thing or set of things, e.g. this unique 20-piece handpainted china dinner service with optional accessories never before offered for sale at such a bargain price.