Halliday & Matthiessen (2014: 455-6):
In parataxis there is no dependence of either element on the other; so there is no ordering other that which is represented by the sequence. This is why we use the numerical notation. … The only modification is that which arises through internal bracketing or nesting…
In a hypotactic structure the elements are ordered in dependence, and this ordering is largely independent of the sequence. Hence we can have various sequences: dependent clause
(i) following dominant,(ii) preceding dominant,(iii) enclosed in or(iv) enclosing dominant …Hypotactic structures may also involve nesting …