Halliday & Matthiessen (2014: 114):
These textual resources are of two kinds: (i) structural; (ii) cohesive. What this means is as follows. The grammar construes structural units up to the rank of the clause complex …; there it stops. But although the grammar stops here, the semantics does not: the basic semantic unit is the text … . So the grammar provides other, non-structural resources for managing the flow of discourse: for creating semantic links across sentences — or rather, semantic links which work equally well either within or across sentences. These latter are referred to collectively under the name of cohesion …