In 2nd degree price discrimination, the producer only knows that there are different consumer groups who have different demand curves. He thus adjusts the quantity/quality of the product in a way that the consumers will self-select into the according quantity/quality-price combination that maximises the firms profit.
Now, I do not understand how the price is being determined.
Specifically, Varian in Intermediary Microeconomics with Calculus (Figure 26.3) writes that consumers will for quantity/quality x01 pay price A which is the area below the demand curve.
Can someone please explain to me how this area below the demand curve can be understood as the price.