The CVV code is the security code on the back of the card that is designed to be an extra layer of security to protect merchants from fraud, especially e-commerce businesses that do not know most of their customers. In that scenario, the cardholder is never face-to-face with the merchant so there is no way to know if they have the physical card in hand. For most legal and accounting firms, the risk for this type of fraudulent activity is low because you know most of your clients and have built a relationship with them.
The security code is recommended but not required in order to process a credit card payment. Not entering the code will have no impact on your processing fees. PCI compliance regulations prohibit ClientPay from storing that information, which is why there are certain areas of ClientPay that either do not display a CVV field or the field is there but not populated and you are unable to enter a code.