Hello Shaun
You must remember that all of these values found within the DS field are simply markings within the IP header. What a device does with them and how it interprets them are up to the administrator of the particular network through which they are being transmitted.
For example, a packet may traverse various networks that are administrated by separate entities.
One such network may be configured to provide higher priority to AF1 than AF4, another a higher priority to AF4 rather than AF1, and another network may ignore these markings all together. The Assured Forwarding PHB definitions do however have an expected default behaviour of vendor equipment for such markings, and these are described in RFC 2597. This document is actually quite an easy read, and describes how these classes are expected to be dealt with by default.
I hope this has been helpful!
Laz