The D4 Premium also features a unique novel ring of blue LED's around the control dial that highlights visual suction speed according to motor speed. Lower models such as the D2 Storm, D2 Titan, and D2 Total only have a single LED next to the rotary power adjustment knob to indicate power on status. The D series features a mechanical piston bag full indicator next to the hose outlet, and 4 swivel castors on the underside for maximum movement in space restricted areas.