Halliday & Matthiessen (1999: 514):
Language itself, of course, is a form of human behaviour, but "languaging" constitutes, for the grammar, another distinct type of process, that of "verbal" (or, better, "symbolic") processes. An act of saying is not simply externalising inner events; it is actively transforming them, into an event of a different kind. It then resembles other semiotic events, many of which do not require a conscious information source (your diary says you have a denial appointment, the light says stop).