Martin (1992: 220):
These staging conjuctions [sic] contrast with developing additives that accumulate messages locally, on topic or within a stage, usually within the same turn. As with external relations the basic opposition is between additive and alternation:
TURN BUILDING:DEVELOPING:ADDITION: SIMPLE[4:137] We could go to dinner;as well we could try and see a movie.
TURN BUILDING:DEVELOPING:ALTERNATION[4:138] We could eat at seven.Or won't you be hungry then?
 In SFL theory, the conjunctive opposition here is simply between two types of extension: positive addition vs alternative variation (see Halliday & Matthiessen 2004: 542).
 The contrast between these "developing additives" and the "staging" conjunctions is thus the conjunctive cohesion contrast between extension and elaboration (and continuity — see previous post), respectively.