Halliday & Matthiessen (1999: 178):
The situation with circumstances and relators is a little more complicated.
(i) Circumstances of the "macro" type are realised by prepositional phrases, which as we have seen are like miniaturised clauses; their components are thus of different types — a (minor) process plus a participant. Circumstances of the simple type, on the other hand, are realised by adverbial groups; these are groups of adverbs, like more soundly, not so very fast, with the single adverb again as the limiting case.
(ii) Relators are typically realised by conjunctions, like and, so, if, that, because, however; these can form groups, such as as if, and yet, but conjunctions are more often expanded by adverbs (just because, even if). In addition there are numerous other types of relator prepositional phrases (in addition, in the event (that), for fear that), nominal groups remaining from earlier prepositional phrases ( [at] the moment (that), [on] the day (that)), and various expressions involving non-finite verbs (supposing (that), provided (that)). The relator construes a logico-semanuc relation between the clauses in a clause nexus (realising a sequence), but it is itself an element in the structure of one or other of the two clauses concerned; e.g. if you have some ink fish preserved in oil, add a few slices at the same time as the halibut.