Not_Registered user is a dummy user and it was created automatically by PayPlans. It is introduced in PayPlans 4 to improve the checkout process. When a guest user tries to subscribe to a plan, he/she will be directed to the Invoice Checkout page without having to login or register. The guest user may continue checking out other plans or register to the site if they wish.
In actuality, the process of checking out requires PayPlans to create a new invoice and subscription. These invoices and subscriptions require a user association. So, a dummy user is needed to tie them together when a guest user checking out a plan.
This dummy user was created once and when the guest user completed the payment process, their subscription will automatically attach to their username.