Would you please at least check Wikipedia before asking a question here? Because if you do read https://en.wikipedia.org/wiki/Uniform_Resource_Identifier, you'll come across such an example in a matter of minutes.

I recall I have tried to check that article but I as a non native English speaker have trouble reading such long articles and even such long opener chapters on a topic I don't know anything about... This is exactly why I asked about an example focally as sometimes the best learning experience is with a good short example.

I have removed the comments that are not relevant. Please use comments to ask for clarifications or generally speaking, to improve the question.

This is a very relevant article on this topic: https://danielmiessler.com/study/difference-between-uri-url/ . A quick answer to your question: a book identifier (ISBN - ...) is an URI which is not an URL.

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