Halliday & Matthiessen (1999: 532):
As far as the textual metafunction is concerned, therefore, any one clause will typically embody two sets of semantic choices. One will be its organisation as a message, a piece of information flowing from speaker to hearer, its limits defined by the speaker’s point of departure and the focus of attention projected by the speaker on to the listener. The other will be the cohesion it sets up with the preceding moments of the discourse, as well as with other discourses and with the total semiotic environment. These enable the clause to function effectively as reflection [ideationally] and as action [interpersonally].