Halliday & Matthiessen (2004: 538-9):
… logico-semantic relations are confined to the internal organisation of each clause complex: the clause complex is the most extensive domain of relational organisation. The cohesive system of conjunction has evolved as a complementary resource for creating and interpreting text. It provides the resources for marking logico-semantic relationships that obtain between text spans of varying extent, ranging from clauses within clause complexes to long spans of a paragraph or more.