For each of the SMS classes, the processing is as follows:

1. If you have an ACS routine in your CDS to determine the SMS class, SMS executes the routine.
2. Next, SMS executes the corresponding ACS installation exit. An ACS installation exit is an assembler language program you can write to perform processing beyond the scope of the standard ACS routines. Such processing might involve:
   • Calling other programs
   • Writing SMF records
   • Writing generalized trace facility (GTF) trace records
   • Performing arithmetic calculations
   • Maintaining large tables of information for quick searches
   • Writing dumps
   • Invoking ACS routines only once
   Each exit can override the corresponding SMS class, whether explicitly specified or previously determined by an ACS routine. The exit can also invoke the corresponding ACS routine a second time, but this does not cause the installation exit to be reinvoked.

   See z/OS DFSMS Installation Exits for additional information about ACS installation exits.

3. Finally, if RACF® is installed and the storage administrator has defined STORCLAS and MGMTCLAS as resources and permitted end users to specify them, the system verifies that the class is defined in the currently active configuration. The system then checks to verify that end users are allowed to use a selected management class or storage class.

For storage groups, SMS invokes only the storage group ACS routine. Storage group does not have a corresponding ACS installation exit.