If no DO STKGROUP statement is specified for a data set, that data set is assigned an unnamed default group, and can only be stacked with other data sets that have been assigned to that default group.
The following steps describe how stacking groups influence the logic used to choose a volume on which to stack a data set:
When a volume is first considered for stacking a data set, Control-M/Tape checks if the volume belongs to the same pool as the data set to be stacked.
Control-M/Tape next checks if the data sets already on the volume belong to the same group as the data set to be stacked (or have a group specification of *ANY).
If the data sets on the volume belong to an appropriate group, Control-M/Tape checks for DO STKRULE statements that might disallow stacking of the current data set with the data sets on the candidate volume.
If no DO STKRULE statements prohibit stacking on the current volume, the data set is stacked on the current volume.
Note: If data sets that are assigned to two different groups are forcibly stacked together on one volume (for example, using Control-M/Tape Batch Stacking utility CTTSBD), no additional data sets can be stacked on that volume (unless *ANY is specified for those data sets).
