Opt-in refers to users actively giving their permission to be notified for specific purposes, such as promotions and marketing. It is considered best practice to seek opt-in permission from users before including them in your messaging campaigns. Earning opt-in permission before messaging is also legally required in many countries for specific channels, although the opt-in process and timing of the request can vary between devices and platforms.
Asking a user to opt-in to specific channel communications reduces the possibility that they will block notifications or uninstall your application. It also leads to higher message click-through rates and conversion rates because users then expect communication from your brand.