Halliday & Matthiessen (1999: 118-9):
Relations of expansion are typically realised in the grammar by conjunctions or conjunctive expressions linking a pair of clauses, either paratactically or hypotactically (e.g. that is, in other words; and, but, or, also, besides; so, yet, then, when, if, because, unless). Some of these may realise more than one category; for example,but may be adversative 'and yet' (extending), or concessive 'and in spite of this' (enhancing);
while may be additive 'and in addition' (extending), or temporal 'and at the same time' (enhancing);
or may be alternative 'or else' (extending: alternatives in the external world, like take it or leave it), or restating 'in other words' (elaborating: alternatives in the world 'internal' to the discourse, like they are reduced to the smallest size, or microminiaturised.).Overlaps in realisation of this kind show that the primary categories we have set up do in fact shade into one another; in particular, extending in some sense occupies a space intermediate between elaborating and enhancing, and shares a fuzzy borderline with each.