The opt-in function is connected with getting permission from the customer to send 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 to communicate special promotional offers, last minute deals or changes in terms and conditions.
Often, the opt-in option is offered through zero rated short codes, meaning the customer can opt-in without any additional price per message. Opt-in is tightly connected with opt-out which is used so the customer can 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. At the end of a message, there are 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.