Halliday & Matthiessen (2014: 436):
… the clause complex is realised graphologically as a ‘sentence’, in the way that this has evolved, over the centuries, as a unit in the written language. The sentence is the highest unit of punctuation on the graphological rank scale and has evolved in the writing system to represent the clause complex as the most extensive domain of grammatical structure. We will use the term sentence to refer only to this highest-ranking graphological unit of punctuation. … Hence in the analysis of a written text each sentence can be treated as one clause complex, with the ‘simple’ (one clause) sentence as the limiting case. … Table 7-3 shows the location of clause complex and sentence in relation to other units and complexes of units in the total linguistic system.