With WhatsApp Business API, messaging has more rules: the customer has to message you first before you’re allowed to message them. You also have to respond within 24 hours to avoid paying a messaging fee.
The within-24-hours-reply is called a “session message” and is free. If more than 24 hours passes since the last message from a customer, your message is called a “template message”. Template messages have to be pre-approved by WhatsApp, and they incur a per-message fee paid to WhatsApp and the partner provider connecting you with the API product. No marketing messages are approved.
While this seems more restricted, Business API suits larger business because of its ability to integrate with partner software. This can extend its features to include e.g. complex customer management features and custom notifications.
It has been announced that WhatsApp Payments, a way to accept payments through the app, will be rolled out in select countries later in 2020. As of yet, businesses can only use WhatsApp for the above purposes.