Halliday & Matthiessen (1999: 566, 567):
At both these strata, models are also located along the cline of instantiation, running between the potential — the overall resources for making meaning, within the context of culture, and the instance — instantial 'texts' constituted of meanings that have been selected from this potential, within particular contexts of situation. The potential end of the cline of instantiation embodies all the contextual-semantic models that the culture embraces. …
Figure 14-1 shows the dispersal of contextual-semantic models along the cline of instantiation. As the figure indicates, an inherent property of instantiation is variation; and scientific models (like other subcultural models) vary one in relation to another. Sometimes they are complementary, sometimes they conflict.