Apple has doled out release candidate builds of iOS 17, iPadOS 17, tvOS 17, and watchOS 10 to developers, in advance of the final formal release of the operating systems to the public.
Developers taking part in the testing program can obtain the latest builds by visiting the Apple Developer Center or updating their iPhone or iPad through the Settings app. Public beta versions generally appear a short time after the developer versions, and participants can sign up through the Apple Beta Software Program website.
Release candidates are generally the last builds of a beta-testing process, and are typically in a similar state to what the final release to the public will look like. While there are occasionally second RC releases, there's typically only one needed.
Just like betas, AppleInsider and Apple strongly suggest users don't install test operating systems or other beta or RC software on "mission-critical" or primary devices, as there is the small chance of issues that could result in the loss of data. Testers should instead use secondary or non-essential hardware and ensure they have sufficient backups of their critical data at all times.