Transactional SMS is a message that needs to be sent immediately, without any delay. This type of message can be used for marketing purpose or customer care purpose. It will include information like weather updates, news, horoscope and etc. I have published here regarding Transactional SMS and Promotional SMS. While Promotional SMS is a message that needs to be sent only if someone has subscribed to your service or product. It will include information about discounts, special offers or new products.