Halliday & Matthiessen (1999: 4):
We might refer to the Hjelmslevian notion of the “content plane” as incorporating both a grammar and a semantics (Hjelmslev 1943). Grammar and semantics are the two strata or levels of content in the three-level systemic theory of language, and they are related in a natural, non-arbitrary way. The third level is the level of expression, either phonology or graphology.