Halliday & Matthiessen (1999: 518):
The participants, on the other hand, which represent the prototype of entities persisting in time and space, are usually not subject to this kind of modification; but they are organised in fairly elaborate taxonomies. These may be construed as systematic relations among different lexical items: thus eyes, nose, mouth, chin are all different parts of face, and lamb, pork , mutton, beef are all different kinds of meat. Something of the same sort happens with verbs, but to a much lesser extent.The other resource for constructing taxonomies of things is the expansion of the nominal group, and here the picture is very different from that with verbs. Nouns are expanded lexically as well as grammatically, so that, while entities (like processes) are located deictically relative to the 'here-&- now', they are also (unlike processes) extensively classified and described.An example such as those two nice colourful picture postcards of Honolulu that Sandy sent us shows these resources at work: cards are classified as postcards rather than, say, playing cards; postcards as picture postcards not plain postcards; picture postcards are described as colourful, and also (signalling the speaker's attitude to them) as nice; they are quantified, as two, and specified deictically as those. Further than that, both a circumstantial feature (of Honolulu) and even an entire process (that Sandy sent us) can be brought in as characteristics which specify more exactly the particular cards in question.Thus the grammar has the potential for construing a complex arrangement of classes and subclasses for any entity which participates in a process; or, on the other hand, of naming it as an individual, by using a "proper" noun instead of a common one. Proper nouns are already fully specific, and hence seldom expanded experientially (they are often expanded interpersonally!); but common nouns are almost indefinitely expandable, and it is this resource which organises our universe into its elaborate taxonomies of things.