Apple iPhone devices have an authentication lock, that is turned on when you first sign an iCloud account into a device without an account, this account is usually one that you use to download apps, or use to sync all of your data with Apple with.
The iCloud authentication lock PERMANENTLY stays on your device until it is MANUALLY removed by signing out of your iCloud account that you put on the phone when you initially signed in, if you do not remove the iCloud account from the phone whenever you sell, or give away your device, the iCloud authentication lock will stay in the phone & when the new user updates, resets, or wants to log into the phone with their iCloud account, it will ask for the password of the account that is already signed into the phone. If for some reason you cannot remember the password to your iCloud account, & cannot recover your account with Apple, the phone the account is signed into is permanently rendered useless, a.k.a. a paperweight.
Once an update or reset has been done on the phone, you will be asked to enter the passcode of the device (if it had one) & afterwards it will ask for the iCloud account that was signed into the phone. You can click here to see what that would look like & contains all our information about unlocking passcode & wanting to use the phone with a different company.
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