For OAM, storage classes separate performance objectives from physical storage and determines where the object resides in the storage hierarchy.
The default storage class for an object is defined in the catalog entry for the object collection to which the object belongs. If the default management class is not available from the catalog entry, OAM uses the collection entry in the DB2 table and updates the catalog entry. The default storage class is assigned by the storage class ACS routine when the first object is stored in the collection.
See z/OS DFSMS OAM Planning, Installation, and Storage Administration Guide for Object Support for a more specific discussion of how to use storage classes with objects.