<xs:attribute name="Function" use="required">
    <xs:annotation>
        <xs:documentation xml:lang="en">Specifies the primary function(s) of a particular Check-in message.</xs:documentation>
    </xs:annotation>
    <xs:simpleType>
        <xs:restriction base="xs:NMTOKEN">
            <xs:enumeration value="SelectCheckInSegments">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Select flight segments for which the passenger(s) will be checked-in.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="SelectPassengersToCheckIn">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Select the passenger(s) to be checked-in.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="AssignSeat">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Assign seat number(s) for the passenger for one or more flight segments.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="ChangeSeat">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Change a previously assigned seat number to a new one.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="RequestUpgrade">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Request for a passenger to be upgraded to a higher class of travel.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="EnterFrequentFlyerInfoBySegment">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Enter frequent flyer details for a flight segment.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="ListForStandby">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Lists a passenger as standby for one or more flight segments.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="RequestForAlternateFlight">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Request for reservation change and check-in for alternative flight segment(s).</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="EnterAllowableSSR_OSI_Information">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Provide Special Service Request or Other Service Information.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="EnterAPIS_Information">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Provide Advanced Passenger Information.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="EnterEmergencyContactInformation">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Provide emergency contact information.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="EnterBaggageInformation">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Provide baggage details such as pieces, weight etc.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="RedisplayEnteredInformationAndStatusOfCheckIn">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Request latest passenger check-in details for one or more flight segments.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="VolunteerForDeniedBoarding">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Identifies a passenger who is willing to be a candidate for denied boarding.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="IssueBagTag">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Print baggage tags for checked-in passengers.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="ReprintBagTag">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Reprint baggage tags for checked-in passengers.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="IssueBoardingPass">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Print boarding passes for checked-in passengers.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="ReprintBoardingPass">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Reprint boarding passes for checked-in passengers.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="PrintItinerary">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Print the passenger's itinerary of flight segments.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="PrintReceipt">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Print a ticket receipt.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="PrintStandbyCard">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Print standby card for a passenger.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="ReprintStandbyCard">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Reprint standby card for a passenger.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="CancelCheckInDuringProcess">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Cancel a check-in transaction in progress.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
            <xs:enumeration value="CheckIn">
                <xs:annotation>
                    <xs:documentation xml:lang="en">Check in passenger without issuing a boarding pass.</xs:documentation>
                </xs:annotation>
            </xs:enumeration>
        </xs:restriction>
    </xs:simpleType>
</xs:attribute>