Halliday (2008: 5,6):
This is the “system” in the sense in which it was formulated and defined by JR Firth (Firth 1957a,b); the system is the paradigmatic relation that “gives value to” the elements of syntagmatic structure. […] It is the system that defines the set of options from which any feature derives its value. […] What characterises the system is the regular proportionality between its terms. The system is closed, so that its terms are mutually defining […]