Halliday & Matthiessen (1999: 123):
As already noted, text organisation also draws on the interaction base; and there are certain parallels with the ideation base with respect to sequences. The interaction base certainly has text forming resources that are uniquely interpersonal. In particular, it has the resources for the collaborative exchanges that are embodied in the notion of interaction — for producing dialogue jointly by means of coordinated moves alternating between the interlocutors.
But these interpersonal moves may also form sequences of moves in a way that is similar to the formation of sequences of figures in the ideation base. Typical examples involve motivating condition ('I invite you to accept x, if you want x'; for example: If you're thirsty, there's beer in the fridge) and evidence ('I think, infer/ you should believe x because y'; for example: John's in Germany because I just talked to him).