The message exit parameter list (MEPL), mapped by IXCYMEPL, or the message notification parameter list (MNPL), mapped by IXCYMNPL, passed to the user routine contains the 16-byte token that the system uses to identify the message or response to be saved. This token is valid only for input to IXCMSGC; once the user routine returns control to XCF, the token is no longer valid. The system also invalidates the message token if the message is discarded or if the Message In service finished delivering all the message data. Once invalidated, the message token will not be accepted by either the IXCMSGC service or the IXCMSGIX service.