cpqDaPhyDrvErrCntlrIndex maps an entry in the physical drive error log back to the controller it belongs to, using the same index value as the controller group. This lets management software join error log entries with the correct controller when a system has more than one array controller installed. For example, on a server with two controllers, an admin would use this index to confirm whether a logged error came from controller 1 or controller 2 before dispatching a technician to the right card.