Halliday & Matthiessen (1999: 603-4):
We have located ourselves, throughout the book, in a certain region of metasemiotic space: that is, we adopted a particular perspective on what we are trying to explain. Our central concern has obviously been with 'meaning': our interpretation of meaning is immanent, so that meaning is inside language, not some separate, higher domain of human experience. …
The central meaning-making resource in language — its "content plane" (within which the ideation base, which has been our focus of attention, is one part) — is stratified into two systems: that of lexicogrammar, and that of semantics.
The semantic system is the 'outer' layer, the interface where experience is transformed into meaning. The 'inner' layer is the grammar, which masterminds the way this transformation takes place.
This deconstrual of the content plane into two strata … is a unique feature of the post-infancy human semiotic, corresponding to Edelman's (1992) "higher-order consciousness" as the distinguishing characteristic of Homo sapiens.