Halliday & Matthiessen (1999: 520-1):
Conceptually perhaps the simplest way of expanding a process is by elaborating on it: saying it over again (or something very like it, with repetition as the limiting case), or else exemplifying it, or clarifying it in some other way. The grammar represents this relationship symbolically in English by prosodic means: the same intonation pattern is repeated, for example we're shut out; they won't let us in. But since this does not appear in writing, various purely written symbols are used instead, typically i.e., e.g. and viz.The second type of expansion consists in extending one process by construing another one as an addition to it (with 'and' as the limiting case); or as an alternative to it, a replacement for it, or as some form of reservation or contrast. Here the grammar typically employs conjunctions, like and, or, but, instead, besides.The third type of expansion is one of enhancing the first process by another one setting up a specific semantic relationship, of which the principal ones are time, cause, condition, concession, and means, Here again the grammar deploys a range of different conjunctions, which mark either the enhancing clause (when, because, by, though, if and so on) or the one that is being enhanced (e.g. then 'at that time', then 'in that case', so, thus, yet ).