Halliday & Matthiessen (2004: 652):
When the realisation of a whole figure is downgraded through metaphor from clause to nominal group, its elements will of course also be downgraded: the process is nominalised and serves as Thing… ; the other elements of the figure are realised either as downranked groups/phrases serving as Qualifier or Deictic… or, by a further step, as words serving as Classifier, Epithet or Post-Deictic.