In this, and also other places, shorthand seams to repeat restrictions and specification elements that come from FHIR profiling. I suggest that Shorthand only defines the shorthand related elements and does not repeat the FHIR specification. This will make it easier for Shorthand to be used and evolve idependently of the FHIR specification version. It will also prevent discrepancies occuring between the two specifcations.

Existing Wording:

The data type choices must always be more restrictive than the original data type. For example, if the parent data type is Quantity, it can be replaced in an only rule by SimpleQuantity, since SimpleQuantity is a profile on Quantity (hence more restrictive than Quantity itself).