Martin (1992: 392):
The modularity imposed by stratification is also an important issue. Discourse systems generate structures which in principle cut across grammatical and phonological ones.
 This continues the misconstrual of strata as modules instead of complementary levels of symbolic abstraction.
 To be clear, in SFL theory, the relation between system and structure is realisation. This axial relation is distinct from the process of instantiation — the selection of systemic features and the activation of realisation statements — during logogenesis.