A URI (Uniform Resource Identifier) is a naming mechanism for Web
resources.
To ensure uniqueness of URIs, it is recommended to use domain names
controlled by the owners.
Different approaches may be used, e.g., URLs, URNs, and FPIs.
A URI may look likes a Universal Resource Locator (URL), but its value
needs not provide a concrete address.
XML public identifiers are constraint only by the characters in use [XML,
production 13].
Examples:
XHTML carries the URI ‘http://www.w3.org/1999/xhtml’. The specifications
for the version 1.0 strict and the transitional variant can be found at the
locations given by the URLs