Why is a PayPal Zettle payment reserved in a customer's bank account?

If a payment is canceled or interrupted, the money might be reserved in your customer's account for a few days. A reservation doesn’t mean that the money will be charged to the account. The customer's bank reserves the money at the time of the purchase to ensure that there is sufficient money to complete the payment.    

The reservation is usually removed after a few business days and the money will appear in the account again.