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2.5 Types "The type system of XPath 3.1 is based on [XML Schema 1.0] or [XML Schema 1.1]." XML Schema 1.0 - XML Schema Part 2: Datatypes Second Edition - http://www.w3.org/TR/xmlschema-2/ XML Schema 1.1 - W3C XML Schema Definition Language (XSD) 1.1 Part 2: Datatypes - http://www.w3.org/TR/xmlschema11-2/ would be more precise references.
Referencing all of XML Schema (Parts 0, 1, and 2) is appropriate in this case.