Device Transfer is the process to transfer your account information from one phone to another phone, or to recover your account information in case something happened to your phone.
Why should we?
There is no telling what can happen to your phone. It can suddenly bust out and requires formatting. When that happens, all your efforts for this game will be lost, even moreso when you realize how harsh this game's summon and progress system is.
But with Device Transfer, you can link your account to a code and recover it any time and in any other devices, allowing you to continue your game progress.
How to do it
Step 1: Issue a Transfer Number
Go to My Room and select the menu Issue Transfer Number (scroll down first). Refer to the image below with the red square.
Step 2: Enter Your Password
Fill in and re-enter your password at the password fields (marked with red squares). Your Password is an important key to transferring your data. Make sure it's something you can remember easily.
Step 3: Get the Transfer Number
Copy the generated Transfer Number labelled within the yellow square (by clicking the Copy button, labelled with red square) and store it somewhere else (Email, notebook, etc).
THIS IS IMPORTANT. This is an equivalent of your 'User ID', different from your Password. If someone else has both this and your password, they can access your FGO account.
Step 4: Enter the Data Transfer Menu
This step can only be done when you are ready to initiate the transfer.
Start the FGO application. In the title screen, the option Data Transfer should be found at the bottom-left corner of the screen (marked with red square). Click it.
Step 5: Fill in Transfer Number and Password
Put the Transfer Number on the top field, and the Password on the bottom field.
Once this is done, your data have been successfully restored or transferred to your new device.
The Transfer Number is valid only for one successful transfer
If you have issued a Transfer Number, you can re-issue a new Transfer Number. After doing the first step, click on the Re-Issue button instead.
Different from a save-state in other video games or emulators, the Transfer Number is not saving the current state of your account, it merely binds your account information to the Transfer Number. Therefore, if you issued a Transfer Number, spent several Saint Quartz, and then initiated Device Transfer, the lost Saint Quartz will not return. On the other hand, if you have received a certain Servant before initiating the Device Transfer, the Servant is not lost.
The transfer cannot be initiated between regions. An NA account cannot be transferred to a JP version of the game