Is it the preference curve in consumer theory or the "equal output curve" in manufacturer theory?
Ridge line and production expansion line are terms used to describe the conditions that enterprises need to meet in optimal production. (1) Ridge refers to the curve that separates the positive slope region from the negative slope region on the equal yield curve. Specifically, the ridgeline refers to the connecting line between zero marginal rate of technical substitution on the equal yield curve and infinite marginal rate of technical substitution on the equal yield curve. The area within the ridgeline is the economic area of production, also known as the relevant section of production, and the area outside the ridgeline is the non-economic area of production. It is called production economic zone because if you choose to produce in this area, it will not waste resources.