Halliday & Matthiessen (2004: 646):
The general tendency is for interpersonal metaphor to ‘upgrade’ the domain of grammatical realisation; for example, while the congruent realisation of modality is a group serving in the clause, the metaphorical realisation is a clause that projects or embeds the clause to which a modal value is assigned. In this way, interpersonal metaphor tends to expand interpersonal systems by adding explicit variants — that is, variants where the subjective or objective orientation is made explicit.