https://www.ibm.com/docs/en/ims/15.2.0?topic=parameters-dfsdfxxx-parameter-procedures
IMS 15.2 - System definition - DFSDF= parameter for procedures
Use the DFSDF=xxx parameter in procedures to specify the 3-character suffix of the DFSDFxxx PROCLIB member that you want to use for this procedure.
imssystemdefinitionparameterprocedures