Halliday (2008: 65):
In a systemic grammar, the most abstract representation takes the form of a system network, modelling the sets of options that the grammar makes available, and the interrelation between one set of options and another: whether they are simultaneous or dependent. The network is a theory of what the speaker can do: what he can mean, at the semantic stratum; what he can say, or “word”, at the stratum of lexicogrammar; what he can say, or “sound”, at the strata of phonology and phonetics. It is a more truly generative grammar, as distinct from the earlier Chomskyan kind of generative grammar which, as Peter Wexler once remarked, should really be called a “contraceptive grammar” since it was all about the enforcement of constraint.