Halliday & Matthiessen (1999: 244):
It seems to us necessary to identify the types of grammatical metaphor and characterise them explicitly in relation to the semantics as a whole. We therefore introduce a general distinction between metaphoric (elements or features) and others. Metaphoric elements, as we said above, are junctional in that they embody a junction of two semantic categories. In the previous chapters, 3-5, we dealt just with elements that could be assigned to a single category: process, thing, quality &c. We shall refer to these as 'ordinary' elements, and contrast them with 'junctional' elements which are those that embody grammatical metaphor. Junctional elements will always have two categories in their description, e.g. 'process thing', 'circumstantial quality', 'relator process'.