Comment:

Reference: add identifier 0..1 Identifier and (maybe type 0..1 code (ResourceTypes) to disambiguate the resource type if the identifier.system does not) to the definition to allow for reference by both id and business identifier (the supplied identifier may be only 1 of the identifiers on the resource instance). Not having such a mechanism means having to create contained resources which complicates the instances especially when there are required elements beyond just the business identifier.

Summary:

Add Reference.identifier and perhaps Reference.type