Halliday & Matthiessen (1999: 528-9):
Since these resources are oriented towards discourse, many of the "textual" systems in any language have a domain potentially higher than the clause and clause complex; they set up relationships that create semantic cohesion, and these ate not restricted by the limitations of grammatical structure.
But they also contribute a critical dimension to the overall grammar of the clause. In this guise, the clause functions as a quantum of information; it is construed as a message, with a range of possible structures providing for different interpretations according to the discourse environment in which it occurs.
These structures have been less fully described than those of the other metafunctions; they were brought to the notice of grammarians by Mathesius and his colleagues of the Prague school in the first half of the present century. But it seems that one typical way of construing the clause as a message is as a combination of two perspectives, that of the speaker and that of the listener (the latter, of course, being also as modelled for the listener by the speaker).