Halliday & Matthiessen (1999: 50-1):
A sequence is a series of related figures. Consequently, sequences are differentiated according to the kinds of relations figures can enter into — temporal (x happened, then y happened, etc.), causal (x happened, so y happened, etc.), and so on … . In any pair of figures related in a sequence, one figure may (i) expand the other, by reiterating it, adding to it or qualifying it; or (ii) project (report, quote) the other by saying it or thinking it. In either case, the two may be either equal or unequal in status, or semantic weight. … Sequences are organised by interdependency relations and they are indefinitely expandable.