This exit is taken from the functional subsystem address space (HASPFSSM).

JES2 gives control to your exit routine after the $JCT has been read into storage, during $JIB initialization processing in the FSMGETDS routine of HASPFSSM if the $JCT read was successful and before initialization of the job separator page area (IAZJSPA) with fields from the $JCT. The $JCT read belongs to the job owning the JOE from which data set(s) will be selected for assignment to the FSA through the functional subsystem interface (FSI) GETDS function.

JES2 can also give control to your exit routine just after the FSMGETDS routine in HASPFSSM reads the JCT for the job owning the $JOE from which a data set will be selected (except if queuing on a setup request) for assignment to a functional subsystem application (FSA).