Halliday & Matthiessen (1999: 320-1):
… both these contextual variables [field and tenor] are, in some sense, independent of language, even though they are constituted in language and the other semiotic systems of a culture. That is, they concern realities that exist alongside the reality created by language itself, semiotic reality. However, there is a third contextual variable that is specifically concerned with the part language is playing in any given context — the symbolic mode, how the linguistic resources are deployed. This covers both the medium (spoken, written, and various subtypes such as written in order to be spoken) and the rhetorical function — persuasive, didactic, informative, etc.