If the DD statement has the SUBSYS keyword and the subsystem-subparameter string contains DDNAME=x anywhere in the string, it is processed as if DDNAME=x was coded on the DD statement. This applies only to the JCL DD statement regardless of the subsystem-name. It does not apply to SVC99 Text Units (TUs).
For each DD with SUBSYS, the exit is given control once just before resource allocation. It is not given control during execution for other events such as OPEN, CLOSE and WRITE.