At the rear, your Smart-UPS VT unit (not included) contains a Power Distribution Unit (PDU). In addition, this PDU may be installed as a second unit on the opposite side. The PDU has six 3-phased breakers. The top breaker (4-pole) protects any external 3-phase equipment, and the (5) other breakers (3-pole) protect the equipment connected to the actual L21-20 power outlets beneath the actual breaker.