Halliday & Matthiessen (1999: 119-20):
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 micro-miniaturised.). Overlaps 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.