With the NOTIFYBY parameter of the XCF Message Out service, the sender specifies that XCF is to give control to the message notify user routine upon completion of the message. When message completion occurs, XCF schedules an SRB to the address space that was primary when the sending member invoked IXCJOIN to join the XCF group. The message notify user routine is called while running under this SRB (comparable to how the message user routine runs). The message notify user routine can be specified either when the member joins the XCF group or when the member invokes the Message Out service for the request.

On entry to the message notify user routine, register 1 contains the address of a message notification parameter list (mapped by IXCYMNPL). The parameter list contains information about the message-out request, a table of information describing the result of the send to each target, and if relevant, information about each response. From within the message notify user routine, an individual response is received by invoking the XCF Message In service to copy the text of the response message from XCF-managed storage to user-supplied storage. See Coding a Message Notify User Routine for detailed information about using a message notify user routine.

It is the responsibility of the sender to ensure that completed messages are processed in a timely manner so as to avoid undue consumption of resources within the sysplex. Failure to do so might cause XCF to reject message-out requests with return code X'0C', reason code X'0C', indicating that there is no user message space left. XCF might also reject a Save Message request of the XCF Message Control service.