Halliday & Matthiessen (2004: 516):
It is always, in fact, a relationship between processes — between a mental or verbal process on the one hand, and another process (of any kind) that is mentalised or verbalised (projected) by it. Nevertheless it is not inappropriate on grammatical grounds to treat some projections as verbal group complexes, on the analogy of the types of expansion to which they are sometimes similar in meaning.