Halliday & Matthiessen (2014: 660-1):
The upper bound of the semantic stratum is, as we have said, the text: this is the most extensive unit of meaning. The upper bound of the lexicogrammatical system is the clause: this is the most extensive unit of wording. In the grammar, there is a single, generalised compositional scale – the grammatical rank scale (clause – group/phrase – word – morpheme); and we can specify not only the upper and lower bounds of this scale – the clause and the morpheme, respectively – but also the intermediate units of patterning: group/phrase and word. But in the semantics, it is far from clear whether there is a single compositional scale: such a scale would have to be generalised across all registerial varieties of a language, but we know that texts vary considerably from one register to another. It is quite possible that different registers operate with different compositional scales; for example, one such scale was identified in the organisation of class room discourse by Sinclair & Coulthard (1975) and another in the organisation of certain types of conversation by Cloran (1994). This issue can only be settled after a great deal more research into the semantics of text has been carried out.