On the VNS page of the wireless controller you have (from top to bottom) : Global , Sites, Virtual Networks, WLAN Sevices , Roles , Class of Service , Topologies .
CoS is Class of Service .
Roles and Class of Service can be configured right on the wireless controller , or on Extreme Management (in the Policy section) and pushed to the Wireless Controller . If you configure CoS first on wireless Controller , it will prevent ExtremeManagement to push and override it . Ideally if you start using Policy from ExtremeManagement , do not touch Roles and Class of Services on the controller - do all you changes on ExtremeManagement Policy instead.