The default unit is an esoteric or generic device name, such as SYSDA or 3390, that applies to data sets that are not managed by SMS. For new data set allocations, the default unit allows users to omit the UNIT parameter from DD statements or dynamic allocation equivalent, as they can when allocating system-managed data sets. The default unit does not apply to objects.

If users do not specify the unit parameter on their DD statements or the dynamic allocation equivalent, SMS applies the default unit if the data set is non-system managed and has a disposition of either MOD (treated as NEW) or NEW. If you specify a default unit in the base configuration, make certain that it exists on the system performing the allocations. If you do not specify a default unit, end users must code the UNIT parameter to allocate data sets that are not system-managed.