A registered letter is one that is recorded and tracked throughout its time in the mail system and requires the mailman to obtain a signature in order to deliver it. Many contracts such as insurance policies and legal documents specify that notification must be in the form of a registered letter. By registering a letter, the sender has a legal document that indicates that the notice was delivered. If you say 'It's in the mail' you can actually prove that it is in the mail. It costs extra for the service.