Question only for Americans!!!!!!!!!!!!?

I think you maybe getting part of a story here. Here's my guess: you were being warned about credit card receipts at bars and restaurants. When you pay for a drink or mail with a credit card, there is a line provided for tips/gratuity. If you leave this line blank, a dishonest server could forge a number of his or her choosing on the line, and your credit card would be charged this amount. An X or a 0 would indicate that you didn't want the tip allied to your card. Either that or someone was messing with you. Either way.