Yes good point. In my case the IDs I want to refer to are much more specific (e.g. registration numbers or VAT numbers for businesses), it is just that referri

Yes good point. In my case the IDs I want to refer to are much more specific (e.g. registration numbers or VAT numbers for businesses), it is just that referring to the registry itself is difficult. So YES: Also in this space there is similar ambiguity -- what is really a "business", what is a "VAT number", in what way does IDs from one registry directly correspond to a concept in another registry. But: one still needs to store and use these IDs (somehow). So my proposal is I guess to simply ignore the classification issue, but still use URNs to solve a subset of the problem. URN already does this, e.g. by allowing use of urn:isbn: or urn:issn:, without one needing to really take a stance about when something is a book, when something is a series, whether something printed can be both. It is enough that the ISBN and ISSN registries exist and you just refer to these registries and that is all you can do. So in your example with regions, an example could be urn:geo:german-city-database:munchen urn:geo:norwegian-town-database:notodden ...where "german-city-database" etc is a placeholder just for any kind of official registry. As long as such registries exist and are recognized as somehow official and given names, that is one step. What you discuss is then I think whether the entries in "german-city-database" and "norwegian-town-database" hold objects of the same "type". That seems like another issue. I think URN could be useful while totally ignoring that issue. There are registries and there are IDs in registries; URN deals only with that; and the problem you raise -- knowing when IDs from a given registry can be used in a given context or not -- must be solved somewhere else.