Halliday & Matthiessen (1999: 384-5):
If we look at logogenesis from the point of view of the system (rather than from the point of view of each instance), we can see that logogenesis builds up a version of the system that is particular to the text being generated: the speaker/writer uses this changing system as a resource in creating the text; and the listener/reader has to reconstruct something like that system in the process of interpreting the text — with the changing system as a resource for the process of interpretation. We can call this an instantial system (see Matthiessen, 1993c).For example, in the course of logogenesis, a recipe is built up as a series of ordered loops through the system of sequences, whereas an encyclopaedic entry may be built up as a systemic taxonomy, developed step by step in delicacy.An instantial system may fall entirely within the registerial system it instantiates; in other words, the meanings created within it may all have been created before. However, it may also create new meanings — new to the speaker and/or listener. In either case, the instantial system is built up successively by the generation process; but as it is developed, it in turn becomes a resource for further instantiation.