Data set stacking is the function used to place several data sets on the same tape volume or set of tape volumes. It increases efficiency when using tape media and reduces the overall number of tape volumes needed by allocation. It also allows an installation to group related data offsite. The data set sequence number subparameter on the JCL LABEL parameter is used in conjunction with VOL=REF or VOL=SER to accomplish this function.

Under certain conditions, SMS invokes the ACS routines more than once. This section describes those conditions as well as the values of the read-only variables for the initial invocation and any subsequent invocations of the ACS routines. Because data set stacking might cause a second or third invocation of the ACS routines, you might want to take special care when using WRITE statements to avoid duplicates in the job log.