Authorization
Authorization is a way to reserve an amount on your customer's EP account. For example, if your customer purchases goods in the amount of $100, then you can send the authorization to the EPP (Electronic Payment Provider) for $100 and reserve that amount. You are not actually charging the account, but are just reserving a specific quantity. Then, if your customer has a $500 available credit before your reservation, she will have $400 available after your reservation.
An authorization amount is valid for a certain period of time. After this time expires, your reservation on your customer's account expires. In this example, your customer's available credit returns to $500.