What does SMS unsubscribe reply R mean?

Users who don't want to receive such short messages can reply "R" to unsubscribe.

In short messages, "reply r" usually means that the receiver needs to reply the letter "r" to confirm the receipt or execution of an operation. In some short messages of services or notifications, users may be prompted to "reply to R to unsubscribe", that is to say, if users do not want to receive such short messages any more, they can reply to "R" to unsubscribe. This is a common way of short message interaction, which is used to manage or customize the received service information.