Halliday & Matthiessen (1999: 117):
Expansion is a highly generalised type of relation, whereby one figure is joined logico-semantically to another figure by a relator to form a sequence of the same order of reality. It will be easier to characterise expansion at the next step in delicacy, in terms of its immediate subtypes: elaborate (reiterate), extend (add), and enhance (qualify):
(i) elaboration is a (partial) identity relation between figures: one is identified with another with a difference in perspective (it matters a lot; it plays an important role) or one is included under another as an example (it plays an important role; e.g., it provides the infrastructure). These are clearly related to one another: identity is the limiting case of inclusion and inclusion is partial identity.(ii) extension is an additive relation between figures: a sequence is made bigger by the addition of another figure. This may involve pure addition ('and': he is too young and he doesn't speak the language) or addition with an adversative feature ('and yet': he speaks the language but he is too young ). As a variant of addition, we also have alternation (he is too young or else he is just immature).(iii) enhancement is a circumstantial or qualifying relation between figures: it is, in a sense, extension plus a circumstantial feature — 'and' + time ('and then', 'and at the same time', etc.), 'and' + manner ('and in the same way', 'and likewise'), 'and' + cause ('and therefore', etc.), etc.: it is autumn, so the leaves are turning brown.