Halliday & Matthiessen (2014: 612, 614):
This interpretation of conjunction as a system embodying two variables brings out the underlying principle. However, the two systemic variables are not entirely independent of one another. The ‘elaborating’ type tends to be ‘internal’ rather than ‘external’, and while ‘extending’ and ‘enhancing’ relations can be either ‘internal’ or ‘external’, particular conjunctions may be either one or the other. Thus when we extend it in delicacy, we would need to take account of the all the possible combinations of type and orientation: (i) a number of conjunctions can be used to mark either external or internal relations, but a number of other conjunctions can be used only to mark either (ii) external or (iii) internal relations. For example:(i) external/internal: and, or, but, however, then, next, so, therefore
(ii) external: just then, previously, soon, meanwhile, next time
(iii) internal: in fact, actually, incidentally, in short, briefly, finally, in conclusion, furthermore, moreover, in this respect, otherwise