The control limit limits the number of data sets or z/OS® UNIX files that can be allocated but marked “not-in-use.”
This control limit is determined by the JCL parameter DYNAMNBR on the EXEC statement and the number of DD statements. If the control limit is exceeded when an application program requests a new dynamic allocation, the system automatically attempts to deallocate enough data sets or z/OS UNIX files to meet the control limit, starting with eligible data sets or z/OS UNIX files that have been not-in-use for the longest time.
If the control limit is still exceeded after all eligible resources have been deallocated, the request for a new allocation fails. In this case, you must explicitly request deallocation of an existing allocation before the new allocation can be satisfied. Refer to the z/OS MVS JCL User's Guide for information on choosing a value for the DYNAMNBR parameter.
Note that if an allocation done with a DD statement is not OPEN, it is considered not in-use and is counted in the current number of not in-use allocations. If it is OPEN, it is not counted in the current number of not in-use allocations.