<xs:element name="MessageFunction" minOccurs="0" maxOccurs="4">
    <xs:annotation>
        <xs:documentation xml:lang="en">Specifies the ticketing function to be done - an ET, itinerary invoice, or an automated MCO or a combination of functions.</xs:documentation>
    </xs:annotation>
    <xs:complexType>
        <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>
    </xs:complexType>
</xs:element>