<xs:attribute name="FlexDatePref" use="optional">
    <xs:annotation>
        <xs:documentation xml:lang="en">To specify which portions of the journey the date is flexible.</xs:documentation>
    </xs:annotation>
    <xs:simpleType>
        <xs:restriction base="xs:NMTOKEN">
            <xs:enumeration value="Outbound">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Date is flexible on the outbound portion.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="Return">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Date is flexible on the return portion.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="Both">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Date is flexible on the entire journey.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
        </xs:restriction>
    </xs:simpleType>
</xs:attribute>