For a ProtectedSpaceExtent that is allocated to a VolumeSet, (i.e. is named in a CIM_VolumeSetBasedOnPSExtent association), ...

For a ProtectedSpaceExtent that is allocated to a VolumeSet, (i.e. is named in a CIM_VolumeSetBasedOnPSExtent association), this property is the number of user data bytes placed on this ProtectedSpaceExtent before moving on to the next ProtectedSpaceExtent in the VolumeSet.  Otherwise, this ProtectedSpaceExtent is considered to be unallocated and this property shall be set to zero (0h.)