xsIDREF
xsIDREF, commonly written as xs:IDREF in XML Schema, is a built-in datatype used to reference the value of an attribute that is declared as xs:ID within the same XML document. The purpose is to model a reference or link from one element to another identified element.
Semantics and validation: The lexical space of xs:IDREF is the set of NCName strings. During validation, the
Usage: In a typical scenario, one element defines an identifier with type xs:ID, and other elements reference
Relation to related types: xs:IDREF refers to a single target ID. For multiple references, the type xs:IDREFS