Overview#Absolute URI is defined in RFC 3986
For example, defining a base URI for later use by relative references calls for an absolute-URI syntax rule that does not allow a fragment.
absolute-URI = scheme ":" hier-part [ "?" query ]
URI Scheme specifications must define their own syntax so that all strings matching their scheme-specific syntax will also match the <absolute-URI> grammar. Scheme specifications will not define fragment identifier syntax or usage, regardless of its applicability to resources identifiable via that scheme, as fragment identification is orthogonal to scheme definition.
However, scheme specifications are encouraged to include a wide range of examples, including examples that show use of the scheme's URIs with fragment identifiers when such usage is appropriate.