Ramifications of *not* constraining the URL in conformance resources to a fixed value (fixedUri or system)?

Solution 1:

Extension.url is defined as having a type of "uri", not "canonical". That means that version-specific references are not permitted. The version of the extension "in play" is driven by the version of the core specification in play and/or the versions of the IGs declared in the CapabilityStatement of the server. If neither of these define the extension, then the version of the extension may be ambiguous. For this reason, its important to not significantly change the semantics of an extension over time - instead, if needed, define a new extension.

There is currently no conformant mechanism of declaring the 'version' of an extension in an instance. If this is an essential requirement for you, submit a change request using the "propose a change" link at the bottom of any page in the spec and the community can discuss alternatives. (Given that Extension is now normative, it is extremely unlikely that changing to allow the 'canonical' syntax will be the agreed solution, as that would be a breaking change.)