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
Common use cases include linking related elements, such as a chapter element referencing a cataloged book element,
XML Schema provides analogous types, namely xsd:ID and xsd:IDREF, which enforce similar constraints within XSD-valid documents.