This counter reports the total amount of space, in megabytes, consumed by this specific snapshot instance, with 4,294,967,295 returned if the value cannot be determined. An admin tracks this per-snapshot usage to identify which individual snapshots are consuming the most resource pool capacity, useful when deciding which older snapshots to delete to free space. For example, one snapshot instance reporting unusually high used space compared to siblings taken around the same time would suggest heavy write activity occurred on the source volume during that snapshot's lifetime.