Halliday & Matthiessen (1999: 293, 294, 289):
This diagrammatic sketch is also intended to show that metaphor involves a mapping from one ideational domain to another, construing a value-token relation between the two (cf. Figure 6-16 above).
For instance, the whole semantic domain of sequences may be mapped onto the domain of figures. The token domain may in turn be the value in a further metaphorical move, such as the move from figures to participants. The metaphorical expansion thus can thus involve multiple planes. …
We also need to take account of local mappings: some more delicate type of figure must be able to be represented by some more delicate type of participant. Thus figures of quality ascription will typically be represented by participants with the quality rather than the process reified as the thing: her speech was brilliant' => 'the brilliance of her speech'.