On some systems, device names follow the format ddcu,
where dd is the device code, c is
the controller designation, and u is the unit
number.
Local Digital Storage Architecture (DSA) devices use a controller
letter of "A" regardless of the physical controller
the device resides on. Preceding the letter "A":
All local DSA disk devices are named
DUAn, where n is a unique
disk unit number.
All local DSA tape devices are named MUAn,
where n is a unique tape unit number.
Use of a single controller letter requires that the unit number
for each local DSA device be unique. Duplicate unit numbers are
possible if the local disks reside on different controllers.
If the system is part of an OpenVMS Cluster environment, device
names are formatted in one of the following ways:
If the device is attached to a single
computer or hierarchical storage controller (HSC) subsystem, the device
name includes the node name in the format node$ddcu,
where node refers to the node name of the system
that the device resides on.
If the device is to be accessed (or dual-ported)
by two computers or HSC subsystems, you must identify the device
with a unique, path-independent name that includes an allocation
class. The allocation class is a numeric value from 1 to 255, which
is used to create a device name in the form $allocation-class$device-name.
For example, the allocation class device name $11$DUA8 identifies
a disk that is accessed by two computers or HSC subsystems, both
having an allocation class of 11.
If the device is connected to a SCSI bus and the
system parameter DEVICE_NAMING is set to 1, you can assign the device
a port allocation class, a number from 0 to 32767 inclusive.
If the device is assigned port allocation class
0, its name will be in the form node $ddcuuu.
The device cannot be located on a SCSI bus that is also attached
to another CPU.
If the device is assigned a nonzero port allocation
class, its name will be in the form $allocation-class$ddAuuu.
The device can be located on a SCSI bus attached to another CPU,
provided the other CPU has assigned the same port allocation class
to the SCSI bus.
For more information about the device name format in VAXcluster
or OpenVMS Cluster environments, refer to OpenVMS Cluster
Systems
.
For information about the naming conventions for Fibre Channel
disk and tape devices, see Guidelines for OpenVMS Cluster Configurations.
HP OpenVMS System Manager's Manual, Volume 1:... 
Managing Peripheral Devices