Halliday & Matthiessen (2004: 312):
… experiential structure … has the function of specifying (i) a class of things … and (ii) some category of membership within this class. We shall refer to the element expressing the class by the functional label Thing. Categorisation within the class is typically expressed by one or more of the functional elements Deictic, Numerative, Epithet and Classifier. They serve to realise terms within different systems of the system network of the nominal group.