During any of the exit routines described in this topic the system might hold an enqueue on the SYSZTIOT resource. The resource represents the TIOT and DSAB chain and holding it or being open to the DD are the only ways to ensure that dynamic unallocation in another task does not eliminate those control blocks while they are being examined. If the system holds the SYSZTIOT resource, your exit routine cannot use certain system functions that might need the resource. Those functions include LOCATE, OBTAIN, SCRATCH, CATALOG, OPEN, CLOSE, FEOV, and dynamic allocation. Whether the system holds that resource is part of system logic and IBM might change it in a future release. IBM recommends that your exit routine not depend on the system holding or not holding SYSZTIOT. One example of your exit routine depending on the system holding SYSZTIOT is your routine testing control blocks for DDs outside the concatenation.