The prompt does two things: 1) It denies the app access to the IDFA identifier that is used for tracking. This is 100% reliable, apps just will not get this if

The prompt does two things: 1) It denies the app access to the IDFA identifier that is used for tracking. This is 100% reliable, apps just will not get this if you click "Ask app not to track". 2) It signals to the app that it is not to track you using any other method either. There is no technical way to enforce this, so it is up to the app developer to honour this themselves. Apple will do some checks to make sure you follow this, but there is obviously no way they can detect this reliably. All they have is the threat of kicking you out of the app store if you get caught.