Data in the DP pool is used via a V-VOL. A V-VOL is a virtual LU to some hosts.
Actual physical space allocated (in 42-MB pool page units) is automatically assigned to a V-VOL from the associated DP pool as the volume's logical space is written to over time. A new V-VOL does not have any pool pages assigned to it. The pages are loaned out from its associated pool to that DP volume until the volume is reformatted or deleted. At that point, all of the volume's assigned pages are returned to the pool's free page list.
This figure shows the relationship between a pool and V-VOLs.