When you specify a secondary allocation quantity of zero for nonstriped VSAM data sets, an extend causes the allocation to occur on a new volume, using the primary-space amount. You can use this feature to spread data over multiple volumes. For striped VSAM data sets, extensions occur by stripe, so if there is a secondary quantity of zero, the primary space value is used for the extensions across all stripes until space is exhausted on the primary and new volumes, or until the maximum extents for the data set is reached.