Halliday & Matthiessen (2014: 655):
While textual statuses are assigned to components of messages, textual transitions hold between whole messages, or groups of whole messages. Such transitions may, of course, be left to the listener or reader to infer, without the help of any explicit markers; but they may also be marked explicitly by means of textual or logical resources. The textual resource for indicating such transitions is the system of CONJUNCTION. Here the type of relation is indicated, but there is no distinction between paratactic and hypotactic relations. The logical resource combines type of relation with TAXIS in the univariate structure of clause complexing.