Halliday & Matthiessen (1999: 605):
This category corresponds to Hjelmslev’s (1943) concept of a “connotative semiotic”: a higher–level system that has language as its plane of expression. These include theories: every theoretical construction, scientific, philosophical, æsthetic, and so on, is a higher–level semiotic realised in language. … From a semantic point of view, such systems constitute contexts for language; they can thus be modelled as part of a general linguistic theory, being interpreted as a stratum of language itself.