The pre-ACS routine exit (IGDACSXT) enables an external source, such as a tape management system, to provide input (through read-only variables) to the ACS routine to influence construct selection and assignment. The tape management system can use these variables to direct new allocations to a particular tape library to coordinate vaulting runs for backups or off-site storage. For more detailed information regarding this pre-ACS routine exit, see z/OS DFSMS Installation Exits.