Halliday & Matthiessen (2014: 585, 586):
Despite the borderline cases, projection is, as we have pointed out, a different kind of relationship from expansion. 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 nexuses, on the analogy of the types of expansion to which they are somewhat similar in meaning. Figure 8-14 gives some analyses for purposes of comparison. Examples under (a) and (b) are analysed (i) as verbal group nexus, (ii) as clause nexus. Those under (c), with that clause, are analysed only as clause nexus, since here the alternative does not arise.