When a CRN is entered, it is checked to ensure that the format is correct (e.g. the correct length, existing port code). If an incorrect CRN is entered based on the format criteria, the customer receives a message to this effect and no fee is charged. As yet, the software does not check whether the CRN actually exists, so if an invalid number is entered by mistake and no discharge confirmation is received in response to the request, after 2 months the customer will receive a notification that the request cannot be fulfilled.