gml:booleanOrNilReason 

Extension to the respective XML Schema built-in simple type to allow a choice of either a value of the built-in simple type or a reason for a nil value.

Simple Type Information

Source 

<simpleType name="booleanOrNilReason">
  <annotation>
    <documentation>Extension to the respective XML Schema built-in simple type to allow a choice of either a value of the built-in simple type or a reason for a nil value.</documentation>
  </annotation>
  <union memberTypes="gml:NilReasonEnumeration boolean anyURI"/>
</simpleType>