TRAI has given directive to all telecom service provider to prefix identification tag, which tell the server provider and from where the message has been originated. This is only for commercial mass SMSes.
Have you received message which have been sent from such codes - AX-Airtel, VX-Citibank etc? Here you may have thought what is the first two characters. First two characters tell the Service provider and service area of the sender. So, A means Service provider is Airtel, and X stands for Karnataka. For all other similar code refer to this page.