Halliday & Matthiessen (2014: 698-9):
As we have seen, the semantic domain of modality is extended through grammatical metaphor to include explicit indications of subjective and objective orientation: a modal proposition or proposal is realised, as if it was a projection sequence, by a nexus of two clauses, or as if it was a fact embedded as a Carrier in a relational clause with a modal Attribute, rather than by a single clause. Here the modal assessment itself is given the status of a proposition in its own right; but because the projecting clause of the nexus is metaphorical in nature, standing for an interpersonal assessment of modality, it is also, at the same time, a modal Adjunct in the clause realising the proposition/proposal.