Key Code Qualifier is an error-code returned by a SCSI device.
This process is part of a SCSI protocol called Contingent Allegiance Condition.
The target will respond to the Request Sense command with a set of SCSI sense data which includes three fields giving increasing levels of detail about the error: The initiator can take action based on just the K field which indicates if the error is minor or major.
The specification for the target device will define the list of possible KCQ values.
Common values are listed below, you should consult your hardware specific documentation as well.