Real User Indication
App developers and websites want to make sure that a real person is accessing and using their systems — that’s the entire point of captchas. The Sign In with Apple white paper reveals an interesting tidbit about how Apple balances this with its pro-privacy stance.
For example, the company uses on-device machine learning to ensure that you are using your device in a way that’s “consistent with ordinary, everyday behavior.” The system then generates a numerical score that’s sent to Apple, which is analyzed to determine if a real person is using the system. It’s an anti-fraud measure with baked-in privacy protections.