A system network is a theory of language as choice. It represents a language, or any part of a language, as a resource for making meaning by choosing. Each choice point in the network specifies (1) an environment, consisting of choices already made, and (2) a set of possibilities of which one is (to be) chosen; (1) and (2) taken together constitute a ʻsystemʼ in this technical sense. This can be read either synoptically or dynamically: either ʻif feature 'a' (and 'b'. . .) is present, then either 'x' or 'y' (or 'z'. . .) is presentʼ, or ʻif you have chosen feature 'a' (and 'b'. . .), then go on to choose either 'x' or 'y' (or 'z'. . .)ʼ.
- Halliday (1985), Pag. XXVII