Halliday & Matthiessen (2014: 392):
But there is a further resource in the nominal group that combines these two potentialities, whereby Head and Thing are both present but are dissociated one from the other, not conflated. What happens here is that one of the pre-modifying functions is taken on by something that is itself a nominal group, in such a way that the Thing gets embedded in a prepositional phrase with of, which then functions as post-Head Qualifier, as in a cup of tea (see Figure 6-11). Of course, the two dimensions of structure are both present throughout; what we are showing here is the way the total meaning is construed by mapping the headhood of cup onto the thinghood of tea.
Where the Head is dissociated from the Thing in this way, it can be conflated with any of the premodifying functions.