This provident fund dragon card of the landlord should be a credit card, right? Is it Tangshan Provident Fund Longka Credit Card? Because only the signature column on the back of the credit card will print 7 digits, the first four digits are the last four digits of the card number, and the last three digits are the security code of the credit card. Is a very important verification code, only on the card, can not be found elsewhere. Generally used for telephone or network authorized transactions. So just like the card number and expiration date, it can't be revealed.