I had a first view into the XML-Schema and Schematron of the ISO-21090 datatypes. I did not find any namespace information. In which namespace do these elements “live” in? Or are they in the “nonamespace”? If so, is there a specific reason for this?
This is called “chameleon namespacing“: the ISO datatypes are not in any particular namespace, but whatever is assigned to them where they are used. Quoting from the standard:
All elements shall be in some namespace, and the namespace shall be defined in the conformance statements of information processing entities that claim conformance with this International Standard. This International Standard reserves the namespace “uri:iso.org:21090” for direct applications of these datatypes such as testing environments
HL7 uses them in the urn:hl7-org:v3 namespace. Other organisations can use them in whatever namespace they want.