What happens when you press “Ask App not to Track”?

Question

I'm reading about Facebook's recent criticism of Apple and starting to wonder what happens when users press "Ask App not to Track". Does clicking that button actually restrict companies' ability to track, or is it just a request? Put short, is there anything stopping companies like Facebook from continuing to track even after the user presses the button?

mtmail · Accepted Answer

Part is technical: App then no longer have access to the device advertisert id. Companies can use that to track you between multiple apps.
Part is transparency and documentation only: Companies need to describe what data is collected and what it's being used for.
https://developer.apple.com/app-store/user-privacy-and-data-...