The "area" is the amount of area left open after being filled with the metering rod. Let's just say .100 jet with a .070 rod would leave you a .030 area. It gives you a comparative chart of what rod and jet combination may best for your setup. Economy is the fatter part of the rod, and power is the skinny part which is when the step up springs push the rods up. The chart is in thousandsth of an inch.