Halliday & Matthiessen (2014: 719):
In our presentation of interpersonal metaphor, we identified a number of common types. 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 (I think ... , etc.) or embeds (it is probable ... , etc.) 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.
In contrast, the general tendency for ideational metaphor is to ‘downgrade’ the domain of grammatical realisation of a semantic sequence, figure or element – from clause nexus to clause, from clause to group/phrase, and even from group/phase to word. Such downgrading affects both the unit whose domain of realisation is downgraded, and the units of which it is composed: the downgrading proceeds down the rank scale by a kind of ‘domino effect’. The downgrading may start with (i) a whole sequence of figures, (ii) with a single figure, or (iii) with a single element within a figure.