There's a process that's loosely described as having one's case reviewed, and then needing to provide detailed information to prove you're the rightful owner of the Apple ID account. "The process is designed to get you back into your account as quickly as possible while denying access to anyone who might be pretending to be you," the FAQ notes.
Not immediately available to all beta testers
Not every account will be eligible to sign up during beta testing. Apple notes, "Individual accounts will be made eligible gradually until we can offer the service to everyone."
If an account is eligible, a user will be alerted after signing in with an Apple ID on a public beta in the Setup Assistant. Apple says users will see a "two-factor authentication" screen if they can opt in.
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