A command device is a LUN that allows for in-band communication and management from a host to the storage array.
The command device is a pass through feature that uses a SCSI device to accept SCSI commands, but instead of being a disk executing reads and writes, custom Hitachi software (CCI, HBSD for Openstack or Tuning Manager for example) encapsulates special commands inside the SCSI payload for provisioning and performance monitoring or replication management, instead of recording data blocks.
host filesystem --> SCSI I/O stack --> Write command--> SCSI disk --> (data recorded)
Command Device (CMD DEV):
host CCI Software--> SCSI I/O stack --> Write command (encapsulated "create replication") --> SCSI disk (that is actually CMD DEV) --> (Array creates replication pair)
Retrieving data ...