When the client clicks on the “Invoice Payment” link, he will see a list of the different payment method options you have already set up at the “Payment Options”.
For online credit card payment methods such as “PayPal”, “Stripe”, etc. Once the client uses one of them, the payment will be automatically processed via the payment gateway, and you will receive a notification in regards to its success.
For other payment methods such as “bank transfers”, “WesternUnion”, “Cheque”, etc., the client should first pay outside the system, and then inform you about the details of their payment on the form.
Once they submit the form with the details, the payment will be added to the system with a “Pending” Status with the details that your client has submitted.
That doesn’t mean that you need to “Approve” the payment to receive it; rather than that, it means that if you have “ALREADY” got the payment outside the system with the details they have submitted and then you should approve it, Otherwise, you will have to contact your client in regards to their submission.