What should a design achieve, and what limits it?

A design has a goal and limits. This shade must cover at least three garden cells. Each cell of shade costs two tokens, and the available tokens are the budget. A good choice must cover enough cells and stay within the budget. The goal is called a criterion. A constraint is a limit that the design must obey, such as a maximum size or budget. A design specification should make success testable and limits explicit. Performance criteria may compete, while constraints such as available materials or a permitted envelope restrict possible solutions. The virtual shade’s coverage and token cost expose that trade-off. These are invented model quantities, not measured properties of real shade structures or materials.