Halliday & Matthiessen (2014: 464):
The combination of elaboration with hypotaxis gives the category of non-defining relative clause (also called ‘non-restrictive’, ‘descriptive’). This functions as a kind of descriptive gloss to primary clause … hypotactic elaboration is a strategy for introducing into the discourse background information, a characterisation, an interpretation of some aspect of the dominant clause, some form of evaluation (as can also happen with paratactic clarification).