“If Barclays receives an instruction, it will make a payment if … the customer has Sufficient Funds. The customer has Sufficient Funds if the cleared balance or any arranged overdraft or limit is enough to cover the payment.”
“If the customer does not have Sufficient Funds for a payment Barclays may treat the instruction as a request for an unarranged overdraft. If Barclays allows the unarranged overdraft, the customer must repay the overdrawn amount on demand.”