Halliday & Matthiessen (1999: 20):
Our [content plane] “sign” is not the Saussurean sign: we are not talking about the relationship between a word and its phonological representation (between content and expression, in Hjelmslev’s terms). The relationship is within the content plane, between a meaning and a wording — the non-arbitrary relationship between the system of semantics and the system of lexicogrammar.