Halliday & Matthiessen (1999: 398):
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:All the categories under this third heading [mode] are second-order categories, in that they are defined by reference to language and depend for their existence on the prior phenomenon of text. It is in this sense that the textual component in the semantic system was said to have an 'enabling' function vis-à-vis the other two: it is only through the encoding of semiotic interaction as text that the ideational and interpersonal components of meaning can become operational in an environment. (Halliday, 1978b: 145)