This defines the scsi logging feature. It is a means by which the user can select how much information they get about various goings on, and it can be really useful for fault tracing. The logging word is divided into 8 nibbles, each of which describes a loglevel. The division of things is somewhat arbitrary, and the division of the word could be changed if it were really needed for any reason. For more info see /usr/src/linux/drivers/scsi/scsi_logging.h