The optional replacing function can be useful for cleanup. For example, you might want to do some cleanup based on your table maintenance or based on the status of your data sets. Possibly you need to do some cleanup based on checkpoints you are tracking, or storage you have freed.

The installation-wide exit currently in use can be replaced by issuing a MODIFY EXIT,ID=ISTEXCDM,OPTION=REPL command. See z/OS Communications Server: SNA Operation for more information about this command. You can replace the exit, but not the exit options. If you want to replace the exit options, deactivate the exit, then reactivate it. If the exit is active when you issue the MODIFY EXIT command with OPTION=REPL specified, the exit is allowed to complete processing before it is driven for the replaced function.

For an exit that is being replaced, VTAM® sets the function code to X'08', builds a parameter list, and calls the exit to indicate to the exit that it is being driven the last time before VTAM replaces it.