Halliday & Matthiessen (1999: 119-20):
In the overview, we introduced two distinct types of structure — the relational organisation of sequences, and the configurational organisation of figures. In contrast to figures, sequences are not constructional units. We can specify the range of projecting and expanding relations available for further developing a sequence, but we cannot specify where a sequence has to come to an end — that is, we cannot specify a sequence as a unit whole with a conventional configuration of parts. Thus if we have expanded one figure we can always repeat the operation:A then BA then B then CA then B then C then D …In contrast, a figure is a unit with a finite number of elements:B follows AHence a sequence can be indefinitely complex, whereas a figure cannot.