In order to use Quality of Service on the traffic for your device, you need to perform a number of configurations.
The order of operations involved in configuring QoS is different from the order in which the interface displays the setting tabs. Not all settings are needed in all cases.
The classes define how network traffic is to be prioritized and allocated.
There are a number of predefined classes, but you can add your own.
In order to direct traffic to the correct classes, you need to define classificaton rules in the Classify tab.
With the classes defined, you can add and order them in a class group in the Group tab.
If you have multiple interfaces, and want different QoS settings for them, you can create multiple class groups.