Halliday & Matthiessen (1999: 528):
The "textual" metafunction is the name we give to the systematic resources a language must have for creating discourse: for ensuring that each instance of text makes contact with its environment. The "environment" includes both the context of situation and other instances of text. Relative to the other metafunctions, therefore, the textual metafunction appears in an enabling role; without its resources, neither ideational nor interpersonal constructs would make sense.