Halliday & Matthiessen (1999: 398):
The textual metafunction second–order in the sense that it is concerned with semiotic reality: that is, reality in the form of meaning. This dimension of reality is itself constructed by [the] other two metafunctions: the ideational, which construes a natural reality, and the interpersonal, which enacts an intersubjective reality. … The function of the textual metafunction is thus an enabling one with respect to the rest; it takes over the semiotic resources brought into being by the other two metafunctions and as it were operationalises them …