Halliday And Matthiessen (2014: 666-7):
… the three subtypes of expansion (elaboration, extension and enhancement) combine with tactic relations to link one clause to another in the formation of clause complexes. The same pattern was found to operate in the formation of group and phrase complexes. In clause complexes and in group/phrase complexes, expansion is manifested within the logical mode of the ideational metafunction.
But we had already met two experiential manifestations of expansion within the system of transitivity. On the one hand, expansion is manifested in the augmentation of the clause by circumstances: these circumstantial augmentations cover all three types of expansion, with enhancement being the most highly developed one. On the other hand, expansion is manifested in the process of a ‘relational’ clause: ‘intensive’ clauses embody elaboration, ‘possessive’ clauses extension, and ‘circumstantial’ clauses enhancement. In the domain of the nominal group, there are also various manifestations of the three types of expansion. Thus the Qualifier may elaborate, extend or enhance the Thing;