Halliday & Matthiessen (2014: 437):
But does the clause complex need to be interpreted as a ranked unit (a ‘sentence’) analogous to the group? We believe the answer is no; the essential nature of a clause complex is brought out by treating it as a univariately structured complex rather than as a multivariately structured unit. In a clause complex, the tendency is much more for any clause to have the potential for functioning with any value in a multi-clausal complex. In other words, the relations among the clauses in a clause complex are generally more like that of a string of nouns such as railway ticket office staff, which could be explained as a (univariate) word complex, than that of these two old railway engines, which could not. We shall assume, therefore, that the notion of ‘clause complex’ enables us to account in full for the grammatical combination of clauses.