Halliday & Matthiessen (1999: 530):
Over and above its contribution to the grammar of the clause, what we are calling the "textual" metafunctional component comprises a further set of resources, which construe clauses and clause complexes into longer stretches of discourse without the formality of further grammatical structure. These are the resources for creating "cohesion". These are of four kinds: reference (sometimes called "phora", to distinguish it from reference as defined in the philosophy of language), ellipsis, conjunction, and lexical cohesion.