Halliday & Matthiessen (2014: 659):
… lexicogrammar has evolved textual resources for creating cohesive links that have the ability to indicate semantic relationships in the unfolding text beyond the domain of grammar. These resources are known collectively as the system of COHESION. Cohesion includes (1) the system of CONJUNCTION for marking textual transitions in the unfolding of text and (2) the systems of REFERENCE, ELLIPSIS & SUBSTITUTION, and LEXICAL COHESION for giving elements different textual statuses in the unfolding of text.