Halliday & Matthiessen (2014: 609):
While the grammar does not provide any grammatical structure beyond the clause complexes, it still provides ‘clues’ to indicate logico-semantic relations operating on any scale within text. This is the cohesive system of CONJUNCTION, which has evolved as a complementary resource to clause complexing: 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.