Provisioned capacity, not used capacity.
Persistent disks bill on the size you provisioned, not the space you filled, and they keep billing while the disk exists — including after the VM attached to it is deleted. Orphaned disks are one of the most common forms of pure waste on a Google Cloud bill.
Billed: Per gibibyte-month PROVISIONED, not used. Rates differ sharply between Standard, Balanced, SSD, and Extreme.