Halliday & Matthiessen (1999: 562, 563):
The ideational meaning potential embodies not one single semantic system but rather several such systems coexisting; in Firth's terms, it is a "system of systems" — in two distinct but related ways [internal complementarities and registerial variation]. …
[Internal] complementarities constitute one form of indeterminacy of the system — one that allows it to be "polysystemic" in the particular sense of embodying more than one way of construing experience.