EXAMPLE:
A URI reference within an RDF graph (an RDF URI reference) is a Unicode string [UNICODE] that:
The encoding consists of:
The disallowed octets that must be %-escaped include all those that do not correspond to US-ASCII characters, and the excluded characters listed in Section 2.4 of [URI], except for the number sign (#), percent sign (%), and the square bracket characters re-allowed in [RFC-2732].
Disallowed octets must be escaped with the URI escaping mechanism (that is, converted to %HH, where HH is the 2-digit hexadecimal numeral corresponding to the octet value).
Two RDF URI references are equal if and only if they compare as equal, character by character, as Unicode strings.
Note: RDF URI references are compatible with the anyURI datatype as defined by XML schema datatypes [XML-SCHEMA2], constrained to be an absolute rather than a relative URI reference.Note: RDF URI references are compatible with International Resource Identifiers as defined by [XML Namespaces 1.1].
Note: this section anticipates an RFC on Internationalized Resource Identifiers. Implementations may issue warnings concerning the use of RDF URI References that do not conform with [IRI draft] or its successors.
Note: The restriction to absolute URI references is found in this abstract syntax. When there is a well-defined base URI, concrete syntaxes, such as RDF/XML, may permit relative URIs as a shorthand for such absolute URI references.
Note: Because of the risk of confusion between RDF URI references that would be equivalent if derefenced, the use of %-escaped characters in RDF URI references is strongly discouraged. See also the URI equivalence issue of the Technical Architecture Group [TAG].
This document [RDF SEMANTICS] does not take any position on the way that URI references may be composed from other expressions, e.g. from relative URIs or QNames; the semantics simply assumes that such lexical issues have been resolved in some way that is globally coherent, so that a single URI reference can be taken to have the same meaning wherever it occurs. Similarly, the semantics has no special provision for tracking temporal changes. It assumes, implicitly, that URI references have the same meaning whenever they occur. To provide an adequate semantics which would be sensitive to temporal changes is a research problem which is beyond the scope of this document.
The semantics does not assume any particular relationship between the denotation of a URI reference and a document or Web resource which can be retrieved by using that URI reference in an HTTP transfer protocol, or any entity which is considered to be the source of such documents. Such a requirement could be added as a semantic extension, but the formal semantics described here makes no assumptions about any connection between the denotations of URI references and the uses of those URI references in other protocols.
The semantics treats all RDF names as expressions which denote. The things denoted are called 'resources', following [RFC 2396], but no assumptions are made here about the nature of resources; 'resource' is treated here as synonymous with 'entity', i.e. as a generic term for anything in the universe of discourse.
The different syntactic forms of names
are treated in particular ways. URI references are treated simply as logical
constants. Plain literals are considered to denote themselves, so have a fixed
meaning. The denotation of a typed literal is the value mapped from its enclosed
character string by the datatype associated with its enclosed type. RDF assigns
a particular meaning to literals typed with rdf:XMLLiteral, described in section 3.