An arranged overdraft is an agreed overdraft facility that's set up on your account. It allows your balance to be overdrawn up to the agreed overdraft limit. You'll be charged interest on the overdrawn balance. You will not be charged if you don't use this facility.
When you don’t have enough cash in your account, or within your overdraft limit, to cover a payment you'll go into an unarranged overdraft. You'll be charged interest on the balance and you'll be charged £5 each time a payment takes you in to, or further in to, an unarranged overdraft. The monthly cap on your Current Account for unarranged overdraft charges and paid or unpaid transaction fees is £75.