Halliday & Matthiessen (2004: 489):
This is the traditional category of ‘apposition’. As with clauses, appositional group or phrase complexes are characterised by tone concord, signalling the semantic relationship of elaboration. The elaborating group or phrase may restate or particularise; restatements include naming, explanatory glossing and shifts in perspective: a number of the themes of elaborating clause complexes are replayed on a smaller scale.