Registered mail is mail for which a signature must be placed before receipt. This can be mail delivered by PostNL but also offered by couriers from for example DHL, FedEX or a bailiff. When you have indicated this at the registered mail settings in the customer portal, mail is accepted by PriPost. An employee will then sign for it. The mail is then processed in the way you have set it up. It is also possible that you indicate that you want to have registered mail forwarded directly as registered mail.