I don't think that the top-level elements in the CDS Hooks defined extension element have ever been required to use reverse-domain-name-notation. It's obviously a good practice. It would be a breaking change if we made it now.  We will add language to the spec: Extension structures SHOULD use a strategy for naming that ensures global uniqueness, such as reverse-domain-name notation, as in the examples below. (Update the second example to be com.example.clientConformance) Add the following STU Note: We seek implementer feedback on whether the recommendation to use namespace-based unique naming in the extension specification should be made mandatory or that we consider adding a mandatory field to extensions that indicates the source/type of the extension (as is done with FHIR).