The type xs:IDREF represents a cross-reference to an xs:ID value. Like xs:ID, it is most commonly used to describe attribute values. Each attribute of type xs:IDREF must reference an ID in the same XML document. For example, the ref attribute of the fnref element in Example B-1 contains an xs:IDREF value (again, assuming it is validated with a schema). Its value, fn1, matches the value of the fnid attribute of the fn element. You can find all the xs:IDREF values that refer to a specific ID using the idref function.

Because xs:IDREF is ultimately derived from xs:string, xs:IDREF values can be compared and used like strings. For more information on working with IDREFs, see "Working with IDs" in Chapter 20.