20 TB traffic per month is provided for each virtual server. Only outgoing traffic is taken into account; incoming traffic is unlimited. In the control panel, you can see the currently consumed traffic. Traffic is rounded down to integers, i.e., if 900 GB of traffic is used up, it will not be displayed. If 1600 GB of traffic is used up, 1/20 TB will be displayed (1 TB from 20 TB are consumed).
On dedicated servers, traffic is unlimited.