CBQoS (Class-Based Quality of Service) is a smart QoS introduced by Cisco, which does classifying incoming data into classes and then applies defined policies on them effectively shaping and prioritizing traffic passing through device.
As those devices are SNMP capable there are OIDs that represent structure how classes and policies are defined together with how many bytes has been classified into each class and how many bytes passed the policy. By constantly polling (each 5 min by default) those OIDs NTA keeps track of bytes transferred per that polling interval.
Rendered in a chart in NTA it could give you basically following information:
- is the QoS configured correctly so it has desired effect on passing data? (there are three cahrts prepolicy - giving you idea how much data arrived, postpolicy - giving you idea how much and how formed in time data look like after policy has been applied, and drops - showing you if any of those data has been dropped due to applied policy)
- also in some cases it can be used as NetFlow replacement if you predefine classes in smart way so it gives you rough idea what data are passing through the device.
I hope that helped you.