idref

IDREF is an attribute type used in SGML-derived markup languages, most commonly in XML documents that are validated against a DTD. It marks an attribute whose value must correspond to the value of some ID attribute elsewhere in the same document, creating a cross-reference between elements.

In a DTD, IDREF attributes reference a unique identifier defined by an attribute of type ID. For example, an element might declare an attribute id of type ID, and another attribute ref of type IDREF to point to that element. There is also IDREFS, a whitespace-separated list of IDs. When a document is validated, each IDREF must match an existing ID in the document; otherwise a validation error occurs. If the document is not validated, IDREF values are treated as ordinary strings.

Common use cases include linking related elements, such as a chapter element referencing a cataloged book element, or a configuration file where items reference definitions declared elsewhere. IDREFs help enforce internal consistency by ensuring references point to real targets within the document.

XML Schema provides analogous types, namely xsd:ID and xsd:IDREF, which enforce similar constraints within XSD-valid documents. Some schema designs additionally use key/keyref constraints to model more complex relationships across elements. Across both DTD and XML Schema, ID enforcement requires document-wide inclusion of matching ID values and is generally limited to references within the same document.