Halliday & Matthiessen (2014: 635):
Reference is a relationship in meaning. When a reference item is used anaphorically, it sets up a semantic relationship with something mentioned in the preceding text; and this enables the reference item to be interpreted, as either identical with the referent or in some way contrasting with it.
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 the wording rather than directly in the meaning.