Halliday & Matthiessen (1999: 518):
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. … 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 it elaborate taxonomies of things.