This DisplayString reports the space currently used on the specific disk identified by multiDiskIndex. Paired with multiDiskSize, it gives the concrete used-versus-total picture for that particular drive, which an admin checks when a specific disk (rather than the whole appliance) is suspected of running low on space. A disk showing usage nearly equal to its total size, while other disks in the same appliance sit mostly empty, would pinpoint exactly which drive needs attention.