Martin (1992: 319-21):
Halliday (1985: 307) treats intensive relationals as elaborating, possessives as extending and circumstantials as enhancing… . This interpretation can be adopted here with two reservations. First, descriptive intensive attributive clauses need to be distinguished from classifying ones:
DESCRIPTIVE : CLASSIFYING ::
Ben is fast : Ben is a champion ::
Ben isn't vain : Ben isn't a Jamaican now ::
Ben was eager : Ben was a go-getter ::
Ben is guilty : Ben's just a victim
The descriptive type will be treated as extending here, since they assign qualities to participants in the same way that Epithet°Thing structures do. The classifying type relates subclasses to their superordinates and is better left as elaborating since its function is similar to that of Classifier°Thing structures in the nominal group.
Second, possessive clauses which have to do with part/whole relations rather than ownership will be treated as elaborating because of their taxonomic focus. Their function is similar to that of the classifying intensives in that they assign parts, building up composition taxonomies for a given field. In this respect they resemble the Pre-Deictic, Pre-Numerative and possessive Deictic structures discussed above.
OWNERSHIP (+) : COMPOSITION (=) ::
Ben has a dog : Ben has strong legs ::
Ben has a house : the house has ten rooms ::
Ben owns a car : the car has four doors ::
the suit belongs to Flo : the suit has only one leg
 As the term 'descriptive' suggests, the expansion type here is elaboration, not extension — just as it is in the case of Epithet and Thing. See previous critique here.
 Part/whole relations — meronymy — are a manifestation of extension, not elaboration. Extension has three main subtypes: composition (meronymy), possession and association (Halliday & Matthiessen 1999: 146), and these are fractal types that are manifested at various scales and domains throughout the linguistic system. To classify composition as elaboration is not only a misunderstanding of the concepts involved; it creates an inconsistency in the model that resonates through the entire system.