Halliday & Matthiessen (1999: 267-8):
…the relators are the most unstable in terms of their susceptibility to metaphoric transformation. They are, as it were, the first to leave, and they travel farther than the rest. We can perhaps link this property of relators to their status in the overall ideation base. Relators construe the highly generalised logico-semantic relations of expansion that join figures into sequences: elaborating, extending, enhancing. […] these relationships of expansion pervade very many regions of the semantic system: they are manifested in the organisation of figures of being, in the types of circumstantial element that occur within a figure, in the taxonomy of ‘things’, and elsewhere, as well as of course in their ‘home’ region of the construal of sequences , as links between one figure and another. That led us to characterise the categories of expansion as “transphenomenal” and as “fractal”: transphenomenal in the sense that they re-appear across the spectrum of different types of phenomena construed by the ideational system; and fractal in the sense that they serve as general principles of the construal of experience, generating identical patterns of organisation of variable magnitude and in variable semantic environments.