Halliday (2008: 97-8):
The nominalisations functioning as technical terms are those which have become systemic; they can no longer be “unpacked” (that is, replaced by a more congruent form). We can say — using a lexical metaphor — that the metaphor is “dead”: a technical term is, essentially, a dead grammatical metaphor. The ones functioning to build up the argument remain instantial; they can readily be unpacked and replaced by an agnate congruent wording.