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ITS-Container.asn contains the PDU header message ID for each message type. Currently this lists up to the "rtcmem" ID of 13.

ItsPduHeader ::= SEQUENCE {
    protocolVersion INTEGER (0..255),
    messageID INTEGER{ denm(1), cam(2), poi(3), spatem(4), mapem(5), ivim(6), ev-rsr(7), tistpgtransaction(8), srem(9), ssem(10), evcsn(11), saem(12), rtcmem(13) } (0..255),  -- Mantis #7209, #7005
    stationID StationID
}

However there are a number of newer message types that don't have a definition even though the specs refer to ETSI TS 102 894-2, this includes CPM, MCDM, VAM, PZM. Whilst these are Release 2 features I don't see where their message IDs are defined, especially since the BTP ports and ITS-AIDs are already defined for these services.