recovery key

See -> Two-step verification for Apple ID

Two-step verification for Apple ID – Apple Support (UK)

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Where should I keep my Recovery Key?

Keep your Recovery Key in a secure place in your home, office or other location. Consider printing more than one copy, so that you can keep your key in more than one place. Your key is easier to find if you ever need it, and you have a spare copy if one is ever lost or destroyed. You shouldn't store your Recovery Key on your device or computer, because that could give an unauthorised user instant access to your key. 

If you ever need a new Recovery Key, find out how to create a new Recovery Key. 

What if I lose my Recovery Key?

You can sign in to your Apple ID account page and create a new Recovery Key using your Apple ID password and one of your trusted devices.

1. Go to appleid.apple.com/uk.
2. Sign in with your password and trusted device.
3. In the Sign-In and Security section, click Recovery Key > Change Recovery Key.

Keep your new Recovery Key in a safe place. Your old Recovery Key won't work any more and you can't use it to reset your password or access your account. If you can't see Recovery Key in the Sign-In and Security section, then you are not using two-step verification. 

FYI: write down your recovery key and keep it in a safe place.

The recovery key is a 28 digit code. It is wherever you keep such items.

Note that the instructions given in a previous post refer to users of two step verification which is an older security system. Most users now use two factor authentication which provides slightly different information. Also note that the link in a previous post takes you to a document that has United Kingdom specific information (URLs for UK users). If you are indeed still using two-step verification, this one should work for wherever you are located --> Two-step verification for Apple ID - Apple Support

Here is the information for two-factor authentication users and the URL will render the version for the country to which your account is set --> How to generate a recovery key - Apple Support

- You can generate a new recovery key from Settings or System Preferences on a trusted device signed in with your Apple ID." See the article for details about how to do this on different types of equipment (Apple equipment only).

- If you forget your Apple ID password and do not know your recovery key, you can try to regain access using your trusted device protected by a passcode.

- When you generate a recovery key, you can't use account recovery ( How to use account recovery when you can’t reset your Apple ID password - Apple Support ).

- "Using a recovery key is more secure, but it means that you’re responsible for maintaining access to your trusted devices and your recovery key. If you lose both of these items, you could be locked out of your account permanently."

- Read the article for more details.