Today card numbers are generated in incremental order per your credit unions increment by number. If you wish to turn on card randomization this will change the middle part of the card number into a completely random number as opposed to an incremental pattern. You also will be able to chose to randomize card expiration dates opposed to the system setting the month the card is created plus the number of months configured to expire. Turning these features on will help make brute force bin attacks more difficult to find valid card numbers.
Note: If your vendor is the same for ATM/Debit and Credit this feature will be turned on for all products as this is enabled by vendor and not by BIN.