Halliday & Matthiessen (1999: 528):
There is a third component in the linguistic construction of meaning; this is what we refer to as the “textual” metafunction. If we were trying to find a term to match the expressions “language as reflection” and “language as action” that we used to gloss the ideational and interpersonal metafunctions, we might come up with “language as information”; but this is itself not very informative. It is a difficult concept because unlike the other two, the textual metafunction has no obviously distinct function at the back of it. All uses of language involve the creation of text. But at the same time this is precisely the context in which the textual metafunction may be understood.