This counter tracks NOT READY SCSI SENSE key errors on this LUN/flow, and per the desc this specifically means the logical unit being addressed could not be accessed when the command was issued. An admin seeing this climb on one flow should check whether that specific LUN is spinning up from a power-save state, is offline for maintenance on the array side, or has failed, since this error is the target explicitly reporting it is not ready to serve the request. A burst of NOT READY errors right after a scheduled array maintenance window would fit expected, benign causes, while a persistent, unexplained pattern would warrant escalation to the storage team.