signing on the back of the card is to ensure the cardholder's safety in using the card. Please be sure to sign it. Just remember how to sign it at will. If you sign the bill incorrectly, the merchant may refuse it. Of course, if your card has a transaction password, or it belongs to a photo card, you can do it without signing it, but this will lose a heavy guarantee.

after signing, don't try to erase it in any way, so that the special paper in the signature column will peel off and the card will be invalid (it is very dangerous if the signature is so easy to modify, hehe).