[1] XQuery 1.0 and XPath 2.0 Data Model (XDM) (Second Edition) ( ( 版)) <http://www.w3.org/TR/2010/REC-xpath-datamodel-20101214/#dt-undefined>
In certain situations a value is said to be undefined (for example, the typed value of an element node). This term indicates that the property in question has no value and that any attempt to use its value results in an error.