Every message sent or received from a user is counted as a Session Message.
When you create a message in a conversation, every image, text, audio, etc. is a message irrespective of the fact that it is in a single "send message" block or multiple blocks or multiple conversations.
A message is counted in your pricing plan only when it is sent or received by a user, i.e. when that conversation runs for a user. If a conversation never runs, the message inside it is not counted.