URIs are specified as being written only in ASCII characters, i.e. the Roman alphabet with no accents plus a few punctuation symbols. For reasons given later (section 4.10), the advice is that only ASCII characters, i.e. only URIs, should be used as persistent identifiers.

URIs can identify anything, including:

• Web documents: The most familiar related term is probably URL, Uniform Resource Location, which identifies a specific information resource on the World Wide Web, such as a Web page, a PDF document, an image etc. URLs all begin with http://.
• Real-world things: The more general term, Uniform Resource Identifier (URI), can identify anything whether it's something on the Web or not.  As formulated in the W3C Note[1]: “URIs identify not just Web documents, but also real-world objects like people and cars, and even abstract ideas and non-existing things like a mythical unicorn. We call these real-world objects or things.”

While a URI has a form of presentation that could be confused with a URL, not every URI is also a URL.