Halliday & Matthiessen (2014: 393):
The relationship the Head and the Thing is either (1) an extending one or (2) an enhancing one: see Table 6-10.(1) When the Head is conflated with the Deictic or the Numerative, the relationship between the Head and the Thing is an extending one: ‘Head is a subset of Thing’.
(2) When the Head is conflated with the Epithet or the Classifier, the relationship between the Head and the Thing is an elaborating one (which is why this type of construction has been called appositive): ‘Head is Thing’.