Halliday & Matthiessen (1999: 293):
Such a representation has to meet the kinds of ideational and textual demands that we have already considered in our representation of examples:
(i) ideational: the representation has to show that metaphor constitutes an expansion of the semantic system. The expansion is an elaborating one, creating chains of token–value relations; and it increases the semantic potential for construing experience.
(ii) textual: the representation has to make it possible to show how metaphorical and congruent variants are given different values in the text base — in the textual semantics.