Halliday & Matthiessen (1999: 117-8):
From one standpoint the limiting case of expansion would be accumulation in a temporal sequence (hence our general term "sequence" for the product of this construal): 'a happens, then x happens'. This gives value to a as the temporal circumstance of x. From this we could derive a wide range of more complex enhancing relations: variations on the simple temporal sequence (after, before, at the same time, immediately after, &c.) and further circumstances such as cause, condition, concession, and their subcategories.
We shall not try to enumerate these here; they are familiar as categories at the level of lexicogrammar (see Halliday, 1985: Ch. 7; Matthiessen, I995b: Ch. 3). These are 'enhancements', multiplying one figure by another, as it were. But figures may also be added to one another, making them part of the same story without assigning any kind of logical priority to either: 'x as well as/instead of/in contradistinction to a'. We have referred to these as extensions. And there is the third type of accumulation where the logical relation is that of 'equals': 'x is the same figure as a'. Here at this end the limiting case is a simple repetition; this may be further elaborated in such a way that one figure is reworded as another, or else further clarified or brought out by an example.