Halliday & Matthiessen (2014: 564):
When groups and phrases are linked hypotactically, they are given unequal status, one serving as the dominant element (α) and the remainder as dependent ones (β γ δ ...). Hypotactic verbal group complexes involve either expansion or projection, but hypotactic nominal group complexes and hypotactic adverbial group/prepositional phrase complexes are based only on expansion.