https://www.ibm.com/docs/en/ims/15.4.0?topic=parameters-usermsgs-parameter-procedures
IMS 15.4 - System definition - USERMSGS=N|Y parameter for procedures
Use the USERMSGS=N|Y parameter in procedure to specify whether (Y) or not (N) the user-supplied user message tables module DFSCMTU0 is to be loaded by IMS.
imssystemdefinition