"In terms of stratification, the book deals with lexicogrammar, the stratum of working. If we use the familiar metaphor of vertical space, as implied in the word 'stratum', the startum 'above' is the semantics, that 'below' is the phonology. We cannot expect to understand the grammar just by looking at i[t] from its own level; we also look into it 'from above' and 'from below', taking a trinocular perspective. But since the view from these different angles is often conflicting, the description will inevitably be a form of compromise. All lingusitic description involves such compromise; the difference between a systemic description and one in terms of traditional school grammar is that in the school grammars the compromise was random and unprincipled, whereas in a systemic grammar it is systematic and theoretically motivated. Being a 'functional grammar' means that priority is given to the view 'from above'; that is, grammar is seen as a resource for making meaning - it is a 'semanticky' kind of grammar. But the focus of attention is still on the grammar itself."
Halliday & Matthiessen 2004 An Introduction to Functional Grammar (3rd edition) p. 31
I choose to think of the trinocular perspective described above with this picture: