2.4.3 Class Selector Codepoints

The DS field values of xxx000 and CU field unspecified are reserved as a set of Class Selector Codepoints. The PHBs which are mapped to by these codepoints must satisfy the  Class Selector PHB requirements in addition to preserving the Default PHB requirements on codepoint 000000.

2.4.4 Per Hop Behaviour Standardization Guidelines

This topic is important with the point of view that the behaviours are one of the important parameters with respect to aggregated flows:

The abovementioned few points are only a chosen few in a long list of PHB guidelines, for more information, there is a complete and comprehensive list of PHB guidelines in [RFC2475].