Halliday & Matthiessen (2004: 532):
… there is a set of lexicogrammatical systems that have evolved specifically as a resource for making it possible to transcend the boundaries of the clause — that is, the domain of the highest-ranking grammatical unit. These lexicogrammatical systems originate in the textual metafunction and are collectively known as the system of cohesion. … There are four ways by which cohesion is created in English: by
(iii) ellipsis [and substitution] and
(iv) lexical organisation.