Information is passed to the ACS routines in the &UNIT ACS read-only variable, so that the ACS routines know when data set stacking or unit affinity is used. In a tape environment, unit affinity is a JCL keyword (UNIT=AFF) used to minimize the number of tape drives used in a job step.
Table 1 shows the values to which the &UNIT read-only variable can be set. The first value is applicable only when UNIT=AFF is used. The others are applicable only when data set stacking is being done, whether or not the UNIT=AFF keyword is present.
| &UNIT Value | ACS Invocation | Data Set Stacking Indication | Device Category of Data Set on Which to Stack |
|---|---|---|---|
| AFF= | First | Unknown | Not applicable |
| STK=SMSD | Second | Yes and different device categories | System-managed DASD |
| STK=NSMS | Second | Yes and different device categories | Non-system-managed DASD or Non-system-managed Tape |
| STK=SMSD or STK=NSMS | Third | Yes and different device categories | Non-system-managed DASD or Non-system-managed Tape |