As described in Coding the Serviceability Calls, the dce_msg_define_msg_table( ) routine can be called by an application to initialize an in-memory copy of its message table, thus freeing the application from depending on its message catalog's being properly installed for its serviceability messages to be properly generated.
However, the serviceability routines will still, by default, attempt first to retrieve a specified message from the message catalog, even if an in-memory table has been initialized; only if the message catalog cannot be found will the in-memory table be used.
You can change the default behavior of the serviceability routines by setting the SVC_NOXPGCAT environment variable to 1 (or any nonzero value). This will force the routines to always go to the in-memory table for the specified message; they will never look for the message catalog.