A business-initiated message is one the business starts with a template; a user-initiated exchange begins when the customer writes first.
On WhatsApp, who speaks first decides the rules. A user-initiated exchange starts when the customer messages you. It opens the customer service window, and you can reply freely. A business-initiated message is one you send without a recent message from the customer, such as a reminder, an alert or an offer. It must use an approved template and the customer must have opted in.
This split exists to protect people from unwanted messages, and it drives cost. Business-initiated templates are charged by category and country; replies inside the window are generally free from Meta.
Plan both directions. Use business-initiated templates for things the customer will genuinely want, and design them to invite a reply, so the conversation can continue as a user-initiated one. Marichi Talks keeps both in one place: templates and campaigns for messages you start, and the shared inbox for conversations customers start.