An Act Now Alert will tell you if your account is going to enter an unarranged overdraft to give you time to pay in money and possibly avoid your payment(s) being returned unpaid and associated charges.
An Arranged Overdraft Alert, we will tell you if you are about to start using your arranged overdraft or, if you spent money during the previous day, that you have started using your arranged overdraft.
This may help you to avoid interest and charges.