Halliday & Matthiessen (2014: 719-20):
In the congruent mode, a sequence of two figures is realised by a nexus of two clauses; but in the metaphorical mode, the domain of realisation is downgraded from clause nexus to clause. In the metaphorical mode, the domain of realisation of either one or both of the figures is, in turn, downranked from clause to group/phrase. These successive steps in downgrading are possible because, as we have seen, both projection and expansion are motifs that are manifested throughout the grammatical system: a sequence of projection can thus be realised not only by the manifestation of projection in the clause nexus, but also by its manifestation in the clause or the group/phrase. The same principle applies to expansion.