Halliday & Matthiessen (2004: 357, 358):
Postmodification is of one type only, namely comparison. As in the nominal group, Postmodifiers are rank-shifted, or embedded; they may be (a) embedded clauses, or (b) embedded prepositional phrases. …This is the only instance of embedding other than in a nominal group. All other embedding in English is a form of nominalisation, where a group, phrase or clause comes to function as part of, or in place of (ie as the whole of), a nominal group.