Halliday & Matthiessen (1999: 124):
We can generalise as follows: the logical resources for forming sequences have evolved in the environment of ideational meaning as sequences of figures. But these highly generalised resources can then also be applied in an interpersonal environment to form interactional sequences. (We should note here, however, that evidence from language development studies suggests that the logico-semantic relations are first construed in interpersonal contexts). In producing a text, we may use either or both, depending on the nature of the text.