The opt-out function is an action where a customer requests the stopping of sending promotional materials and messages to his or her's email, mobile number or chat application.
In the messaging world, it is mostly used by marketing departments as a way of offering customers the ability to unsubscribe from the messages he receives.
The practice differs around the world and in some countries the opt-out function is a mandatory part of message text by law. It usually comes at the end of a message, as instructions on how to opt-out. Chat applications like Viber have an opt-out option for A2P messages at the top of the chat group.