Sales tax execution

C form is in the context of interstate sales. It is issued by the sales tax department to a Registered Sales tax payer(here the buyer). The advantage of C form is the buyer will be charged at a concessional rate(normally 2%) against the actual CST rate(Say 14.5%) in that state by submiting the c form to seller

Normally, if u buy the goods from out of state, then u will be charged with CST(say 14.5%) by the seller on ur Invoice amount at the prevailing rate in such state. 

If the buyer obtained C form from the department and issued to Seller in other state( he must be a registered under sales tax  dealer), then the seller will charge at a concessional rate(say 2%). And the seller should submit the same to the department in his jurisdiction.