In Surf, you can add a comment to your transfer. This comment will be stored on the Free TON blockchain and can be open or encrypted. While viewing transactions, you can view the comments to them.
If you add an encrypted comment, then on the blockchain it looks like a line that cannot be read. But in Surf, you and your recipient can read the comment.
The comment increases the transaction fee as it increases the size of the transaction itself.
To add a comment to the transfer in Surf:
Fill in the Message or Memo field on the Send screen.
Check the box next to the Encrypt note to encrypt the comment and make it unavailable to third parties.
Review the fees in the Network fees field at the bottom of the screen and click the Confirm and Send button.
Confirm sending the message with a PIN, fingerprint or face recognition.