<simpleType name="NullType">
    <annotation>
        <documentation>

        If a bounding shape is not provided for a feature collection, 

        explain why. Allowable values are:

        innapplicable - the features do not have geometry

        unknown - the boundingBox cannot be computed

        unavailable - there may be a boundingBox but it is not divulged

        missing - there are no features

      </documentation>
    </annotation>
    <restriction base="string">
        <enumeration value="inapplicable" />
        <enumeration value="unknown" />
        <enumeration value="unavailable" />
        <enumeration value="missing" />
    </restriction>
</simpleType>

• NullType
• gml:null