@siljelb to confuse things a bit more 
I guess you might have noted that URNs are also URIs (but not necessarily resolvable). Feel free to help me/us think about how they are best used in openEHR, see:
- https://openehr.atlassian.net/wiki/spaces/spec/pages/786956289/DV_EHR_URI+related+issues
- It has long been on the todo list to finish and submit a URN registration for openehr, here is a registration template https://openehr.atlassian.net/wiki/spaces/spec/pages/1913782313/IANA+URN+namespace+application+for+openEHR
P.s. also somewhat related: Usage of OID, URI (URL/URN) in DV_IDENTIFIER fields?