about "C form"

hi, our company do business in india, and I am a accoutant. I saw the rate of CST on one of our sale bills is 10%, I think if the customer give us " C form" ,the rate of CST will be 3% or 2% ,according to the invoice date. But someone told me sometimes the customer can not give us the "C form" . For example , if we bought goods from Bombay, we sell these goods to the company located in Delhi,and our company registered in U.P. ,so that customer can't give us " C form", because this business involved three states , so no " C form" . is that truth? please help me , thinks!