Martin (1992: 334):
With instantial relations, the elaboration marker = will be used as a valence, classifying the relation as one between message parts rather than within:
[4:2] 9.r. With the smaller breeds of dog such as Corgis, all the Toy-breeds, Dachshunds and this type of thing we — as our turn comes,
The lexical relations between the various canines here are general, not instantial, since each is 'a general fact about English' rather than being 'entirely specific to a single text' (Hasan 1985: 81).