Server Configuration > User and Group Permissions > Defining a User in your Realm > Key Considerations
Key Considerations
When changing a user’s name in the GUI, change the name in the database before the user logs in using the new name so that all items with that assigned user are updated throughout the database.
You should not use commas in user or group names. If you use commas in the user name, you cannot select the user in a multi-valued user field.
If you are creating or editing e-mail notification for specific users, you must deny the ModifyMyNotification permission for those specified users. This ensures those users cannot modify their own notifications while you are making modifications.
Do not re-use existing user names. For example, re-using an existing user name from a different domain can cause login errors and change historical data attributed to the existing user name.