What the hell does "certified letter" mean?

[2naSalit]

12. There's registered and certified

as per the Google...

With Certified Mail service, the sender receives a mailing receipt, and the mail recipient must sign for the envelope or package. The USPS keeps a copy of the recipient's signature on file as proof of delivery for two years from the date of mailing. ... With Certified Mail, you can't purchase insurance

Registered mail is a mail service offered by postal services in many countries, which allows the sender proof of mailing via a mailing receipt and, upon request, electronic verification that an article was delivered or that a delivery attempt was made.

What is the difference between registered and certified mail?
The main difference between certified and registered mail with the U.S. Postal Service lies in the level of accountability, with registered mail being tracked throughout the entire process from the departure point to the destination.