When you code DISP=NEW or DISP=MOD, space is allocated to system-managed multivolume (non-extended-format) data sets with the guaranteed space attribute in the storage class, as follows:

1. Initially, primary space is preallocated on all the volumes.
2. When the primary amount on the first volume is used up, a secondary amount is allocated on the first volume until the volume is out of space or the data set has reached its extent limit.
3. The preallocated primary space on the next volume is then used.
4. When the primary space on the next volume is used up, a secondary amount is allocated.
5. Secondary amounts continue to be allocated until the volume is out of space or the data set extent limit is reached.

All succeeding volumes follow the same sequence.