Halliday & Matthiessen (2014: 435):
(1) In terms of metafunction, it is organised by the logical mode of the ideational metafunction, contrasting with circumstantial augmentations of the clause (experiential) and cohesive sequences (textual). …
(2) In terms of rank, it is located at the highest rank of the grammar — clause rank; and it is thus related to the clause in terms of logical complexing rather than in terms of experiential constituency. …
(3) In terms of stratification, the clause complex realises a semantic sequence of projection or expansion; and it is, in turn, realised by a sequence of tones in speech and by a sentence in writing.