Short answers, pulled from the story.
Economic theory requires that products share identical or similar performance characteristics, serve the same occasion for use in time and place, and be offered within the same geographic area where transport costs do not block access. Without all three conditions holding true, economists will not classify the pair as close substitutes.
Cross-elasticity of demand measures responsiveness by calculating the percentage change in quantity divided by the percentage change in price. A positive cross-elasticity value confirms that goods are substitutes because higher prices for one drive buyers toward the other.
Perfect substitutes exhibit linear utility functions with straight indifference curves where consumers base decisions solely on price differences. Imperfect substitutes display curved indifference lines representing slight characteristic differences that cause the marginal rate of substitution to vary depending on the starting point of the exchange.
Good X acts as a gross substitute for good Y if spending on Y increases when the price of X rises, but this relationship lacks symmetry since Y might not be a gross substitute for X. Net substitutability introduces a constant utility function where income effects disappear through hypothetical intervention to ensure that if X is a net substitute for Y, then Y is also a net substitute for X.
Michael Porter identified the threat of substitution as one of five critical industry forces where customers can easily forgo buying a company's product if close alternatives exist nearby. High risk emerges when switching costs remain slight between available options and low brand loyalty makes consumers sensitive to even minor price changes.