Utility type for null elements. The value may be selected from one of the enumerated tokens, or may be a URI in which case this should identify a resource which describes the reason for the null.
<simpleType name="NullType"><annotation><documentation>Utility type for null elements. The value may be selected from one of the enumerated tokens, or may be a URI in which case this should identify a resource which describes the reason for the null.</documentation></annotation><union memberTypes="gml:NullEnumeration anyURI"/></simpleType>