Halliday & Matthiessen (1999: 68-9):
So far we have sketched the most general organisation of the ideation base as a typology of semantic types or classes. This is one theoretical perspective on the organisation — a perspective realised in certain conventional systems of representation, such as the system network. It allows us to bring out quite clearly the global organisation of the ideation base as a resource for construing experience; and it also allows us to show how the semantic types in this global organisation are interrelated, as in the case of participants filling Actor and Goal rôles in figures of doing.
At the same time, there is another theoretical perspective on the organisation of meaning — the topological perspective. Here meaning is construed in terms of a spatial metaphor: we can view the ideation base as an elastic, multidimensional semantic space. This metaphor is already familiar in discussions of meaning; we find it in Trier's notion of semantic fields, in the distinction between core meanings and more peripheral meanings, in specifications of semantic distance, and so on.
The notion of a vowel space (with its 'cardinal vowels') provides a familiar analogy. As a material construction, it is limited to the three dimensions of physical space; but as a physiological space, it accommodates variation along a number of dimensions, and brings an elasticity to the expression plane that is in some extent analogous to the metaphorical elasticity that we are ascribing to the plane of the content.