Username in App42 API calls should be a unique handler to identify each app user separately. If you are registering your user with his emailId, you can use that as user name, or if he is registered with facebook, you can opt for using facebook id of user as username in App42 API call. This way you can manage uniqueness of user in your app. One user can have many score Ids so score Id can not be used as uniqueness for user.
Let me knwo if it helps.