Halliday & Matthiessen (1999: 566):
At both these strata [context and semantics], 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 a culture embraces.