Halliday & Matthiessen (1999: 204):
Similarly, those of the extending type may also function in either of these two grammatical rôles [as Thing or as Facet]. As Facet, they specify some quantity (either by container, e.g. a jar of jam, or by division, e.g. a piece of cake), some aggregate (e.g. a crowd of onlookers), some aspect or component (e.g. the other side of the argument, the top of the mountain, the trunk of the tree), or something added or substituted (e.g. an extension of your ideas, the latest addition to your family, an alternative to this proposal). As Thing, again, they are participants, typically concrete objects (e.g. a glass jar, the top [=lid] of the canister, build an extension on the property).