Halliday & Matthiessen (1999: 119-20):
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 … . In contrast, a figure is a unit with a finite number of elements … . Hence a sequence can be indefinitely complex, whereas a figure cannot.