Martin (1992: 230):
Unlike CONJUNCTION, which is typically realised as textual Theme in clause initial position, CONTINUITY is realised rhematically. Since all CONTINUITY items except already can also function as conjunctions, this difference in realisation proves useful in focussing on the differences between the two systems.
 When cohesive conjunction is marked by a conjunction, a structural element in the clause, it is obligatorily textual Theme. On the other hand, while conjunctive Adjuncts, realised by adverbial groups or prepositional phrases, are characteristically thematic, they also occur elsewhere in the clause. See Halliday & Matthiessen (2004: 83).
 The neutral clause position of mood Adjuncts (Martin's 'continuity items') is just before or after the Finite, but they also occur before a thematic Subject, as interpersonal Theme, and at the end of the clause as Afterthought (Halliday & Matthiessen 2004: 126). Genuine markers of continuity, continuatives, occur as textual Themes.
 To be clear, 'conjunction' is a word class, not a function. Words of other classes do not function as conjunctions.