Halliday & Matthiessen (2014: 491-2):
Embedding is thus the ‘rank shift’ by which a clause or phrase comes to function within the structure of a group. The characteristic function of an embedded element is as Postmodifier in a nominal group … . Other functions are: as Head of a nominal group (ie as a nominalisation); and as Postmodifier in an adverbial group. … These are summarised in Table 7-12. All embedding falls into one or other of these major categories; there are no further types. It should be remembered that the category of nominal group includes those having adjective (Epithet) as Head.