Halliday & Matthiessen (1999: 521):
In principle, any particular type of expansion or projection can be interpreted in either way, either as “paratactic” or “hypotactic”; but in fact there is some degree of partial association: certain combinations are favoured, and others corresondingly disfavoured. For example, in English, when one process is construed as a simple restatement of, or addition to, another, the two are likely to have equal status; whereas where one is seen as enhancing the other they are usually unequal — a means is secondary to what has been achieved by it, a cause is secondary to its effect.