This states the RAID role this specific referenced disk plays, for example a normal data member, a dedicated hot spare, or a global hot spare, within the virtual drive it belongs to. An admin checking this on a manually-mapped disk reference confirms the drive in this slot is actually contributing the intended function, since assigning spare instead of normal to a slot meant to hold data would silently shrink the RAID group's usable capacity. A role of dedicated hot spare here would mean the disk in this referenced slot rebuilds only this virtual drive's group if a member fails, not any other RAID group on the controller.