When a customer adds a credit card to Stax Bill, a validation is requested to the gateway in order to check if it is a real card and able to transact. For most gateways, this will be a specific validation request to the gateway. This can sometimes take the form of a $0 charge or a $1 charge and void.
If a card cannot be validated, then it will not be stored in any vault and the customer will need to check and add new payment method information.