URL and URI are different things (like the linked document mentions). Often, URL is what is meant, so write "URL". But, in some cases (such as for namespacing),

URL and URI are different things (like the linked document mentions). Often, URL is what is meant, so write "URL". But, in some cases (such as for namespacing), URI is what is meant. My own opinion is, write URL if it is meant to access or reference a file or service or something, or URI if it is only an identifier for a namespace or something like that (i.e. if what it points to (if anything) is generally irrelevant). Usually, URL is correct (even though it is a URI too). However, a domain name is not a URL (nor a URI); it is only a part of one. (Also, some people say that if you want purely identifiers, to use UUIDs instead of URIs. But, a UUID is a URI, too! Prefix "urn:uuid:" in front to make a UUID into a URI.)