Full policies can be reviewed here, but we just wanted to give you a couple guidelines.
Subscriptions cannot be used for ads or promotional content.
If you choose to make a subscription, consider it like information that users have solicited, like news, productivity or personal trackers.
Subscriptions must be initiated by the user (opt-in).
You can send out initial requests for subscriptions. However, if the user opts out, that's it. Consider creating a recipe with buttons that will allow them to subscribe later.
The 24-hour policy
You cannot push a message out to users after 24 hours have passed following their last interaction with the chatbot, unless you are attempting to follow up on the conversation: in this case, one and only one message is permitted by Facebook. No spamming!