For a system-managed tape request, by default, the JCL UNIT specification is ignored. This enables the automatic class selection (ACS) routines to steer allocation to a set of eligible libraries and devices and enables MVS allocation to randomize across the eligible devices. This is the preferred and recommended way to use the system-managed tape support. There may be times, however, when it is beneficial to direct an allocation request to a specific device or set of devices. You can specify the JCL keyword SMSHONOR on the UNIT parameter or the dynamic allocation text unit DALSMSHR. When this keyword is specified, MVS allocation will use the list of eligible devices returned by SMS and the devices specified on the UNIT parameter and will allocate using the intersection of the two lists. So, if a single device is specified, and that device is one of the eligible devices returned by SMS, MVS allocation will allocate to the device specified. And if an esoteric is specified on the UNIT parameter (one or more devices), MVS allocation will allocate from the intersection of the two lists. Devices that are not in both lists will be ignored. For more information on the SMSHONOR keyword, refer to z/OS MVS JCL Reference.