the VPU allows unobstructed sighting and easier operation. The VPU is attached to the front of the VREU (Variable Exhalation Resistance Unit) and can be removed. The pass-through lead interfaces with the mask connector and houses a second connector to attach additional communication equipment while the VPU is attached. The unit requires a single Li-Ion battery - CR123 (not included). The microphone is powered by the VPU.