The username is used to identify a person. If it is unmistakable, it is the user ID. This is used to authenticate and log the user into computer systems and networks. The user ID must be entered in combination with a password when an account is created. Once an account has been created, the user can log in to his bank or Internet account again and again.
User names are unmistakable and are assigned once only. This means that there are never two people with the same name. If a user name has already been assigned, another user with the same name must choose a changed name variant.