Halliday & Matthiessen (1999: 384):
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 call this an instantial system.