Source : Free On-Line Dictionary of Computing

tagged queueing
     
         A method allowing a device or {controller} to
        process commands received from a {device driver} out of order.
        It requires that the device driver attaches a tag to each
        command which the controller or device can later use to
        identify the response to the command.
     
        Tagged queueing can speed up processing considerably if a
        controller serves devices of very different speeds, such as an
        {SCSI} controller serving a mix of {CD-ROMs} and high-speed
        {disks}.  In such cases if a request to fetch data from the
        CD-ROM is shortly followed by a request to read from the disk,
        the controller doesn't have to wait for the CD-ROM to fetch
        the data, it can instead instruct the disk to fetch the data
        and return the value to the device driver, while the CD-ROM is
        probably still {seeking}.
     
        (1997-07-04)