Below the sentence, the typical relationship is a constructional one, of parts into wholes. In functional grammar this means an organic configuration of elements each having its own particular functions with respect to the whole (most elements in a grammatical structure are multifunctional). One manifestation of this structural relationship is the sequence in which the elements occur; but this is only one variable among others. Into this constructional type of organization are introduced two minor motifs: (1) structural patterns of another kind that are more like the dynamic processes of text formation [...] and (2) non-structural forms of organization that create cohesion – reference, ellipsis and so on [...] Above the sentence, the position is reversed. Here the non-constructional forms of organization take over and become the norm, while only in certain cases, particular kinds of text, are there recognizable units like the structural units lower down. And the sequence in which things occur is no longer a variable available for realizing functional relationships, like Subject before or after Finite verb; it becomes a dynamic order determined by the semantic unfolding of the discourse.
- Halliday (1985), a pag.XXI