The QoS (Quality of Service) extension module enables bandwidth limitation and prioritizing for different kinds of traffic through the firewall. You can also set priority bits (TOS or DSCP) in the packets to force prioritizing in the receiving network equipment. For each interface you can state a guaranteed and a maximum bandwidth for the total outgoing traffic of this interface and for classes of traffic. The traffic can be classified on sender, receiver, whether it is SIP traffic or not, DSCP and TOS marking, type of service (e-mail, www, ftp etc) and packet size. You can also prioritize traffic based on the traffic classes.
A service is defined as an IP protocol and, where it is applicable, sender and receiver ports (TCP, UDP) or types (ICMP).