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.
<simpleType name="stringOrNilReason"><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 string anyURI"/></simpleType>