If the business is initiating the conversation and there is no open customer care session, then it must always begin with sending an approved message template. This will begin a business-initiated conversation in the same category of the message template (either utility, authentication, or marketing), and you will be charged based on the category of the conversation.
If there is no active conversation in place, but there is an open customer care session, the business can respond using a non-templated message. This will begin a service conversation (referred to as a user-initiated conversation prior to June 1, 2023). During a service conversation, only non-templated messages can be sent. If you send a template message, this will initiate a new conversation that corresponds to the category of your template (authentication, utility, or marketing), and you will be billed accordingly.
Learn more about the Sinch WhatsApp Business API.