RFC 2597 Assured Forwarding defines four classes for queuing purposes in addition to three levels of drop probability for each queue. AF defines 12 DSCP values and their meanings. the names of hte AF DSCPs conform to the format: AFxy
Where x implies on of four queues, and y implies one of the three drop priorities.
AF PHB suggests the higher value of x, the better queuing it should receive. Also the higher value of y the worse the drop treatment is for those packets (higher probability of being dropped).
The binary version of AF DSCP show the patterns of the values, the first 3 bits designate the queuing class and the next two bits designate the drop preference, allowing tools that operate only on IPP to react to the AF DSCP values making it backwards compatible with non-DiffServe nodes.