Physical and logical device names

    You can use both physical and logical device names to refer to tape
    drives and disks.  The physical device name is the hardware name for
    the device, such as DUA0:, MUA0:, and so on.

    A logical name is a name you associate with a device.  For example,
    you might associate the logical name USER$DISK with the hardware
    device name DUA0.  You create a logical name with a DCL command before
    running BACKUP Manager.  For example:

        $ DEFINE USER$DISK DUA0:

    Using logical names provides two advantages.  First, you can pick a
    name that is meaningful to you.  Second, you can change the device
    associated with the logical name.

    How might this be helpful?  Assume that the logical name USER$DISK
    was associated with DUA0.  You and other system users could then use
    USER$DISK in file names, command procedures, and so on.  If you then
    move the data from DUA0 to a larger disk, for example, named DUA1,
    you only need to associate USER$DISK with the larger DUA1 disk.  The
    system users could continue to use the USER$DISK logical name without
    having to be aware of the change.