Authorisation Only (Auth Only) is a special type of sale transaction. It authorises an amount on a customer’s card but the item does not settle until a later time, sometime several days or weeks later. The purpose of an authorisation only transaction is to reserve an amount against a cardholder’s available credit limit for a certain period of time. For example, you may perform an authorisation only transaction if an item ordered is out of stock. When the item becomes available, you settle the transaction, charging the card at that time.