In a WAX flow, your WhatsApp messages go through two types of nodes: the Template node and the Reply node. Here's what each one does and when to use it.
Template node: to start the conversation
The Template node sends a message approved by Meta: a promotional offer, an order confirmation, a cart reminder… It's what starts the conversation with your customer.
Reply node: to continue the conversation
The Reply node sends a free-form message (text, image, buttons) with no Meta approval. It can only be sent after your customer takes an action, meaning the customer:
clicks a Quick Reply button in your message;
sends you a message (a question, a keyword…).
Once they've done that, you can chain as many Reply nodes as you need.
The Reply node is available on WhatsApp only. It doesn't exist for RCS or SMS.
Special case: a flow that starts with an incoming message
If your flow is triggered by an incoming message (inbound trigger, for example a keyword sent by the customer), the customer has already written first. You can reply directly with a Reply node, without a Template node.
What you can't do with a Reply node
Start a conversation yourself (campaign, automation without a customer message): you need a Template node.
Connect a button that redirects to an external link, starts a call or copies a promo code to a Reply node: the connection shows in red and won't work.
In the editor, a red line between two nodes means the connection isn't possible, and an orange line means it's possible but not recommended. Everything is explained here: Why do red or orange lines appear between two nodes?