For SMS-managed data sets, DFSMShsm obtains a list of the management classes that are active in the system, unless a volume function is processing or has just completed. See DFSMShsm retrieval of management classes and storage groups for a discussion of the exception. It then compares the time since the data set was last opened to the value of the LEVEL 1 DAYS NON-USAGE attribute in the appropriate management class for each data set. If the time since the data set was last opened is equal to or more than the value in the LEVEL 1 DAYS NON-USAGE parameter, the data set migrates to ML2. Otherwise, the data set remains on the ML1 volume. If the value of the LEVEL 1 DAYS NON-USAGE attribute is NOLIMIT, the data set remains on the ML1 volume.