Annular 2 vs 3 circuit refers to the addition of an intermediate circuit. It is a discharge tube placed about half way between the venturi, minor diameter of the throat, and the throttle shaft. It adds the most fuel at part throttle, but still contributes at WOT. It was designed to be used for an individual runner application, when pulses from the runner are so high they disrupt part throttle booster operation. 4 and 6 cylinders can benefit even with a plenum as pulses are strong. I have found only rare occasions that need a third circuit.