A storage class is a list of storage objectives and requirements. Each storage class represents a list of services that are available to data sets and objects having similar access requirements. A storage class does not represent any physical storage, but rather provides the criteria that SMS uses in determining an appropriate location to place a data set or object.

In general, SMS attempts to select a location that meets or exceeds the specified objective, but it does not guarantee response time. If no location satisfies the performance objective, SMS attempts to find a location that most closely matches the specified objective for DASD data sets. If there is more than one device of similar characteristics to choose from, SMS selects the volume with the most available space for a majority of times.

Only you, as a storage administrator, can define or alter the storage classes for an SMS complex. End users can specify but cannot override storage classes assigned for a data set by the ACS routines.