Halliday & Matthiessen (2004: 561-2):
Another form of anaphoric cohesion in the text is achieved by ellipsis, where we presuppose something by means of what is left out. Like all cohesive agencies, ellipsis contributes to the semantic structure of the discourse. But unlike reference, which is itself a semantic relation, ellipsis sets up a relationship that is not semantic but lexicogrammatical — a relationship in wording rather than directly in the meaning.