Halliday & Matthiessen (1999: 127):
Sequences at lower ranks than that of figures retain their logical mode of realisation in the grammar. Expansion and projection thus flow throughout the system, forming sequences. This is in fact an instance of a general principle: expansion and projection are trans–phenomenal categories in the sense that they are manifested over the system as a whole — not merely in different logical environments across ranks, but also experientially. […] This feature is particularly exploited when the system is expanded through grammatical metaphor.