Halliday & Matthiessen (1999: 512):
Textually, the grammar is the creating of information; it engenders discourse, the patterned forms of wording that constitute meaningful semiotic contexts. From one point of view, therefore, this "textual" metafunction has an enabling force, since it is this that allows the other two to operate at all. But at the same time it brings into being a world of its own, a world that is constituted semiotically. With the textual metafunction language not only construes and enacts our reality but also becomes part of the reality that it is construing and enacting.