Halliday & Matthiessen (2014: 549):
At the same time as it contributes to the rhetorical-relational organisation of text, the clause complex also serves as a domain of organisation within the textual metafunction. We have seen that from a textual point of view clauses serve as messages – as quanta of information in the flow of discourse; based on this insight into the textual nature of the clause, we can characterise the clause complex as a message complex.
Blogger Comments:
From a theoretical point of view, this might be seen as a poor choice of term, since 'complex', in every other case, is applied only to rank units — to forms, not functions — and concerned with the logical metafunction, not the textual metafunction.