Halliday & Matthiessen (1999: 12):
The text base provides the resources that enable the speaker to produce contextualised discourse and to guide the listener in interpreting it. These include resources for engendering a wide variety of diverse rhetorical structures, for differentiating among the different values and statuses of the components of the unfolding text, and for ongoingly expanding the text so as to create and maintain the semiotic flow.