This not_registred is a dummy user and it is created by PayPlans itself. In PayPlans 4 we have improved the checkout process.
When a guest user tries to subscribe to the plan, the user will directly reach the Invoice Checkout page without having to login or register.
Here, on this page, users can register/login as well as check out for further processes. But in actuality, to show this Invoice Checkout page, PayPlans creates a new invoice and subscription. Invoices and subscriptions should be associated with any user, but we do not currently have any user information.So, we have created a dummy user named not_registered for it.
This dummy user is created only once. When users completed the payment process, the subscriptions will automatically attach with their user names.