The American Innovation and Choice Online Act seeks to regulate the conduct of large online platforms by making certain discriminatory practices unlawful. It defines a "systemically important platform" as one controlled by a person with at least $175 billion in average annual gross revenues and a significant user base, specifically 34% of the U.S. population over 12 or 34% of U.S. households, over a sustained period. This designation subjects these dominant platforms to new rules designed to foster a more competitive online environment. The bill prohibits several key actions by these platforms. These include preferencing their own products or services over those of other business users, limiting business users' ability to compete or access platform features and data, and tying access to the platform to the purchase of other services. Furthermore, platforms are forbidden from using nonpublic business user data to compete against those users, restricting users from changing default settings that steer them to the platform's own offerings, or applying terms of service in a discriminatory manner. Retaliation against users or business users who raise concerns about legal violations is also prohibited. Platforms can assert affirmative defenses, such as demonstrating that the conduct was necessary to comply with law, protect safety, privacy, or security, or prevent fraud, provided it was consistently applied, narrowly tailored, and not a pretext to disadvantage competitors. Another defense is proving the conduct did not materially harm competition. Enforcement of these provisions falls to the Federal Trade Commission, the Attorney General, and State Attorneys General, who can bring civil actions in federal court. Remedies include substantial civil penalties, ranging from 1 to 10 percent of the platform's total U.S. revenue during the violation period, as well as injunctive relief and potential forfeiture of executive compensation for repeat offenders. The bill also mandates expedited antitrust proceedings for cases involving systemically important platforms. The legislation includes important rules of construction, clarifying that it does not require platforms to divulge intellectual property, share data with foreign adversaries, or impose liability solely for offering end-to-end encrypted messaging or subscription services. Actions taken to protect intellectual property rights, such as copyright and trademark, are also not considered unlawful. The Act is set to take effect one year after its enactment.