Apple will change you authentication in iOS 11 and Mac OS High Sierra (via iMore)

Apple users will automatically be changed to two-factor authentication (2FA) when implementing iOS 11 and Mac OS High Sierra, so those using two-step verification (2SV) will notice a change. In the new authentication process, 2FA will send verification codes to all trusted devices (rather than just one trusted device in 2SV), you will not need to keep a copy of the recovery key, and there is a way to see where (approximately) the sign-in is taking place.  To learn more check out iMore's article here.

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