Apple prepares RC version for upcoming releases. Xcode 13 RC is also released.
Developers could use Xcode 13 RC to submit apps to App Store targeting new OS releases.
Apple is expected to release new OSes on Monday, September 20th. Release date for macOS is not yet known.
RC download: Beta Software Downloads
Reference: Developers can now submit iOS 15 and iPadOS 15 compatible apps to the App Store, Apple Seeds iOS and iPadOS 15 Release Candidates to Developers, Apple Seeds Release Candidate Version of watchOS 8 to Developers, Apple Seeds tvOS 15 Release Candidate to Developers, iOS 15, iPadOS 15, watchOS 8, and tvOS 15 Will Be Released On September 20
Developer:Prepare apps for QA and submissions.
QA engineer:Verify apps built with Xcode 13 RC.
PM/DM:Prepare apps for submission.