Board Thread:Questions and Answers/@comment-4802891-20160816043558/@comment-26137249-20160817181851

user id (friend code) is the primary key for every account on database, if you lost it then its hard to recover the account.

And if you check the Account Recovery page's info, you must submit your user id, and if you read the comment then you will find lots people neglect this and its hard to proof that its your account without it if you're free to play, because if you're already paid user then you're safe because you can use the receipt info as proof