Halliday & Matthiessen (2014: 30-1):
But the grammar also shows up a third component, another mode of meaning that relates to the construction of text. In a sense this can be regarded as an enabling or facilitating function, since both the others — construing experience and enacting interpersonal relations — depend on being able to build up sequences of discourse, organising the discursive flow, and creating cohesion and continuity as it moves along. This, too, appears as a clearly delineated motif within the grammar. We call it the textual metafunction.