The use of URI in this specification, as opposed to URL, was a matter of W3C policy; W3C is actively pursuing a policy of generalizing pointers and popularizing the use of the term URI, with the goal of actually popularizing their use.
This was somewhat controversial, since at the time of the creation of the XML spec, virtually every computer in the world was equipped with a URL resolver, whereas virtually none were properly equipped to deal with URIs in the general case. Furthermore, the number of people who really understand what "URI" means is infinitesimal, compared to the number with a good working understanding of URLs.
Concretely, a URI (Uniform Resource Identifier) is either a URN (Uniform Resource Name) or URL (Uniform Resource Locator). URLs are well-known, but contain more subtleties than one might think. URNs are an attempt to give something on the Web a longer-lasting and more robust name than a URL offers. A good introduction to all this stuff may be found here.
Note the consistent usage of the word Resource; a resource is a key concept in the architecture of the Web: any addressable unit of information of service. In practical terms, the definition (although useful) is somewhat circular: a resource is anything that can have a URI, and a URI is a short piece of text that identifies a resource.
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