This read-only integer is the logical index number the operating system assigns to a Clustered RAID Array Controller, unique per physical controller. An administrator uses it as the key to correlate a specific controller's serial number, firmware, and condition entries across the CPQCR-MIB's controller table. Two controllers in a clustered pair would show up here as separate index values, such as 0 and 1, so their statuses can be tracked independently.