For information regarding the credit system in WHMCS refer to this page.
Tick to enable the add funds system allowing customers to buy credit held in their account which can then be used to pay invoices. Invoices generated after credit has been added will automatically be used towards paying the balance. Both you and your customers can manually use the credit to pay invoices generated before credit was added.
The other settings define the amount a customer can add in a single transaction and the maximum limit of account credit.
Require Active Order
We recommend keeping this option ticked so clients can only add funds to their account once at least one order has been manually approved. Unticking this option can lead to an increased risk of chargebacks and fraud as malicious users can add funds - and therefore pay for products - without having to pass any fraud screening measures.