Halliday & Matthiessen (2014: 471):
The markers of paratactic extension are prototypically markers of extension; that is their core use – and, or, but, nor; in contrast, the markers of hypotactic extension are of mixed origin: most of them seem to have been pressed into service from other areas of the grammar – enhancing binders (while, if in if ... not (... then)), linkers followed by that (except that, but [for the fact] that) and conjunctive prepositions and preposition groups (e.g. besides, without, apart from, instead of, other than), and the two additive markers of finite clauses (while, whereas) are used both in the sense of ‘and’ (additive: positive) and in the sense of ‘but’ (adversative).