The description at http://hl7-fhir.github.io/operations.html#2.2.0.6 is complex and under-documented. It's hard to follow without examples. To my mind, it would make more sense to allow asynchronous to define their own semantics, rathAdder than trying to identify a single pattern by which the a given operation can be called in two different ways (synchronous + asynchronous). So, for example, I might define a synchronous $calculate-riskoperation and an asynchronous $calculate-risk-async operation (and the latter might take a parameter like "callback-url". These kinds of details can be operation-specific, rather than one-size-fits-all, without sacrificing interoperability.