As you know Facebook Messenger Platform doesn’t allow to send message after 24 hours of last interaction by any subscriber.

Once any subscriber interacts with your Messenger, you have 24 hour time to send promotional message as many as you want (Don’t spam).

After 24 hours, you are not allowed to send any message (Except using some specific tag only for non promotional message.)Here One Time Notification API is provided by Facebook to send promotional message after 24 hours window.

Source : Messenger Platform's One-Time Notification API (Beta) allows a page to request a user to send one follow-up message after 24-hour messaging window have ended. The user will be offered to receive a future notification. Once the user asks to be notified, the page will receive a token which is an equivalent to a permission to send a single message to the user. The token can only be used once and will expire within 1 year of creation

Why it’s so important ? After deprecated sending message after 24 hours, OTN is the only option to send promotional message now. OTN message sending is happened in two steps. First you will need to set bot that asked for permission to send future promo message regarding any specific offer. Later, you will be able to broadcast message to those people, have given permission for sending message. So, asking this permission & Opt-in by subscribers are super easy. Users just need a click to get subscribed for it. And that’s guaranteed & fully proved that Messenger has almost 95%+ open rate. So it’s hundred times better than sending email broadcast. Just imagine, if you have 500 users subscribed for Black Friday offer or any other promotional campaign, once you broadcast them, almost 450+ people will open it.