When the Auto-Refill option is ON, you can also activate Bill Protection. It allows you to control your data usage and prevent excessive charges.
With Bill Protection, a charge couldn’t be more than the amount you set personally. You will be notified when you reach 85% and 100% of the Bill Protection amount. When you reach 100% of the amount, your Line gets deactivated and you need to refill the Line manually via your account.
If you have more than one Line, you can turn ON/OFF the Auto-Refill and Bill Protection for each of the Lines separately. You still can turn ON the Auto-Refill option without activating Bill Protection.