We say that the operation returns value sets, but there's no indication of whether the value sets are expected to contain embedded expansions or not, nor if there will be situations where expansions are not present, whether there's an expectation that the payer system must support the $expand operation or whether there's a possibility the expansion might be done by a different server - and if so, how the app knows what server(s) to try.  This should be made explicit in the spec.