Halliday & Matthiessen (2014: 659):
… in the course of unfolding of text, lexicogrammatical selections create logogenetic patterns at all ranks. This is patterning in the text that has nothing to do with composition or size: instead of composition (the relationship between a whole and its parts), the patterning is based on instantiation (the relationship between an instance and a generalised instance type). The patterning represents a slight move up this cline from the single instance to a pattern of instances, as in a news report where one projecting verbal clause after another is selected until this emerges as a favourite clause type. The logogenetic patterns that emerge as a text unfolds form a transient system that is specific to that text; but from repeated patterns over many such transient systems may, in turn, emerge a generalised system characteristic of a certain type of text or register.