The backends are used for adding new users to organizations, either by way of self-registration or invitation by existing organization users. The default backends should suffice for simple implementations. If you make use of a profile model or a user model other than auth.User you should extend the releveant backends for your own project. If you’ve used custom URL names then you’ll also want to extend the backends to use your own success URLs.
A registration backend is used for creating new users with new organizations, e.g. new user sign up.
An invitation backend is used for adding new users to an existing organization. The backend should add existing users and create new users.