Facebook Messenger's messaging policy is best described as complex, the different types are best described below
a. Standard Messaging - With this businesses can only send messages within a sliding 24 hour window that resets every-time the customer sends a message to the business. One message outside the window is allowed to follow up on the conversation, alternatively business can choose to send sponsored messages to customers outside the 24 hour window. Learn more about 24 hour window here.
Interesting thing here is that the messages can be promotional, although within that 24 hour window.
b. Subscription messaging(Beta) - Subscription messaging is limited to certain use cases right now that are enlisted here. These messages can be sent anytime outside the 24 hour window but message content cannot be promotional or be an advertisement of any sort.
c. Sponsored messages - These messages are basically one more type of paid ads on Facebook which can be promotional or non promotional. These messages should comply with Facebook's other advertisement policies. Get more details here.
IMIConnect by default gives you access to standard and subscription messaging via your Facebook pages linked in your IMIconnect account. But, we expect that business adhere to the guidelines for sending promotional content as described above.