In case of using version GPT 3.5-Turbo (which is default) one message credit is one message sent by the chatbot to the user. If the user sends 2 messages to the chatbot and the chatbot sends 2 messages back, then 2 message credits are used. In case of using version GPT 4-Turbo, one message uses 10 message credits, in case of GPT 4, one message uses 20 credits.