The user ID or UID is what the system really uses to enforce permissions on files and processes. If two users have the same UID, they will be able to access each others files - so every user should have a unique UID. When creating a new user, Webmin will fill this field with a UID that is not in use by any other user.

The `root` user always has a UID of 0. If you create another user with UID 0, that user will have the same powers as `root`.

If you are editing an existing user and change the UID, the Change user ID on files? option determines which files (if any) have their ownership changed.