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TestFlight makes it easy to test beta versions of iOS, tvOS, and watchOS apps and App Clips, then provide valuable feedback to developers before they release their apps on the App Store.
Developers invite testers through email or a public link.

When you test a beta app or App Clip, Apple collects and sends crash logs, usage information, and any feedback you submit to the developer. The developer may use this information to improve their app and related products. Apple may use crash logs and usage information to improve our products and services.

For instructions guide on installation, testing, and feedback submission and for details for tips on how your data is handled.