The CS# is a priority level in the DiffServ Code Point (DSCP) system of QoS. The AF# and EF values are priority levels as well. There are best practices for each priority level based on throughput and delay needs of different applications. A use for AF# is to place conforming traffic in for example AF21, if the monitored traffic reaches a threshold (Exceed threshold) you can mark it down to AF22, and then if the traffic reaches another threshold (Violate threshold), you can mark it down to AF23. Your traffic would then have different drop precedences depending if it hits the queue at AF21, AF22, or AF23. This type of classification would need to take place as close to the source of the traffic as possible and then the queueing policies would be enforced at an aggregation point.
EF is Expedited Forwarding - For delay sensitive traffic like voice. This traffic is usually placed in a priority queue.
CS0 Selector 0 (CS0) Best Effort
Class Selector 1 (CS1) Scavenger
AF11 Bulk Data
Class Selector 2 (CS2) Network Management
AF21 Transactional Data
Class Selector 3 (CS3) Call Signalling
Class Selector 4 (CS4) Streaming Video
AF41 Interactive Video
Class Selector 5 (CS5)
Expedited Forwarding (EF) Voice
Class Selector 6 (CS6) IP Routing