Halliday & Matthiessen (2014: 611):
The logico-semantic relation is marked by a conjunction — either by a non-structural one that is used only in this way, i.e. only cohesively, such as for example, furthermore, consequently; or by a structural one whose prototypical function is to mark the continuing clause in a paratactic clause nexus. The former serve as conjunctive Adjuncts and are very commonly thematic; the latter are simply analysed as structure markers and are obligatorily thematic as structural Theme.