Letter Sealing is a feature that provides End-to-end encryption (E2EE) for chat room messages. E2EE is a communication system designed so that messages saved on our servers are encrypted and cannot be read by anyone except the sender and receiver of the message.
Once the sender and receiver have both turned on Letter Sealing, they will be able to start sending messages, but with added security.
If you use LINE for PC with Letter Sealing enabled on the smartphone version of LINE, you will be required to verify your identity the first time you send a message. For information on how to verify your identity, see this Help article.