Apple iPhone 15 models are displayed at a store on the products’ release day in Los Angeles on September 22. Patrick T. Fallon/AFP/Getty ImagesAs the case moves forward, expect Apple to challenge a vital component of the government’s lawsuit: How it defines the markets that Apple has allegedly monopolized. The US government claims Apple has monopolized two distinct markets: A market for smartphones, and a separate market for “performance smartphones.”Market definitions are critically important to antitrust lawsuits, because the courts can only rule on illegal monopolization claims with…