We included this discussion [https://chat.fhir.org/#narrow/stream/179166-implementers/topic/Definition.20pattern] in the resolution of this item We have discussed this subject before but have never considered that the url is the identifier of the RECORD of the NamingSystem and that seems to make sense. Previously we assumed that one of the uniqueIdentifiers in the NamingSystem represented the canonical identifier for the system, but as the discussion shows, this is confusing when the list of uniqueIdentifiers change and the "preferred" identifier changes. In this case this should be a change in the list of identifiers, but this should NOT change the canonical url for the NamingSystem, since that is possibly used by other artifacts to refer to this NamingSystem "artifact" or record. Hence, we believe .url should be separate and different from uniqueIdentifier.preferred. We will add .url 0..1 (to be consistent with CodeSystem and ValueSet) and add narrative to explain that this represents the unmutable canonical url for the instance of the NamingSystem record that contains identifiers for the naming system. We will also include guidance that this canonical url SHOULD not be one of the identifiers in the list. Also, update the examples.