This attribute reports the used storage capacity of the ZPOOL in percentage.
ZPOOL is the ZFS storage pool. It is a collection of devices that provide physical storage. The devices that are used to form the collection could be either physical or virtual devices. The total storage capacity available in the storage pool is the combined storage capacity of all the devices in the storage (depending on whether or not the mirroring is done). You can create various file-systems on top of this virtual storage pool, ZPOOL. These file-systems (ZFS) share the storage capacity of the pool. Thus, the combined usage by all ZFS file-systems does not exceed the storage capacity of the pool. Hence, it becomes very important to monitor the storage pool capacity than the capacity of individual ZFS file-systems created on top of the ZPOOL.
The used storage space is the resultant of the shared storage space by all file-systems created on top of this pool.