Halliday & Matthiessen (2014: 549):
The clause complex is, as we have emphasised, the most extensive domain of grammatical patterning — of patterns of wording, patterns organised in terms of logical, recursive systems and structures. It makes a major contribution to the organisation of text, serving to realise (rhetorical) sequences within (rhetorical) paragraphs. In other words, it contributes to the rhetorical-relational development of text by providing grammatical resources for ‘choreographing’ local rhetorical complexes. The grammar also provides resources for guiding the development of text beyond the domain of the clause complex, but these resources are concerned with cohesion rather than with structure.
The term 'paragraph' might be seen as a poor choice of term for a semantic unit, not least because it is not mode-neutral. A paragraph is an organisation of written language, and would be more appropriately applied to a unit on the stratum of graphology.