<xs:attribute name="Function" use="required">
    <xs:annotation>
        <xs:documentation xml:lang="en">Specifies the ticketing function to be performed.</xs:documentation>
    </xs:annotation>
    <xs:simpleType>
        <xs:restriction base="xs:NMTOKEN">
            <xs:enumeration value="ET">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Issue an electronic ticket.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="ItineraryInvoice">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Provide an itinerary invoice.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="AutomatedMCO">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Issue an automated miscellaneous charge order.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="Interface">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Send request to back office accounting.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
        </xs:restriction>
    </xs:simpleType>
</xs:attribute>