Halliday & Matthiessen (1999: 126):
To say that these relations are between figures means in principle that they hold between figures as a whole; that is, given a pair of related figures the domain of expansion or projection relation between them is each of the figures in its entirety. But in some instances, some subdomain may be particularly implicated. This is perhaps especially true with elaborating sequences — the grammar of hypotactic elaborating clause complexes tells us as much: the elaborating dependent clause includes a relative reference expression (if the clause is finite) and the clause is placed immediately after the domain that is being elaborated, whether that is the whole clause […] or some element within the clause […].