Halliday & Matthiessen (1999: 288-9):
The metaphorical relation is thus similar to inter-stratal realisation in that it construes a token-value type of relation. Here, however, the relation is intra-stratal: the identity holds between different meanings, not between meanings and wordings. The metaphor consists in relating different semantic domains of experience: the domain of figures is construed in terms of the domain of participants, and so on (just as in a familiar lexical metaphor the domain of intensity is construed in terms of the domain of vertical space). It is the fact that metaphor multiplies meanings within the semantic system that opens up the possibility of metaphorical chains, with one congruent starting-point and another highly metaphorical end-point (A"' stands for A" stands for A' stands for A; e.g. 'engine failure' stands for 'the failing of an engine' stands for 'an engine failed'). The semantic system is being expanded along the dimension of the metaphorical token-value relation; but the expansion is still within the semantic system itself.