Which DSCP per-hop behavior is split into subclasses based on drop probability?

Assured Forwarding (AF) is a per-hop behavior (PHB) that allows for differentiated services by defining four classes, each with three drop precedences (low, medium, high). This means that within a given AF class, packets can be marked with different drop probabilities, allowing for more granular control over congestion management. For example, AF41 (class 4, low drop probability) will be dropped less readily than AF43 (class 4, high drop probability) under congestion. Expedited Forwarding (EF) prioritizes traffic to minimize delay and jitter, but it doesn't have subclasses for drop probability; it's treated as a single, high-priority class. Default Forwarding (DF) is the standard best-effort service, with no specific guarantees or subclasses. Class-Selector (CS) PHBs are backward-compatible with IP Precedence and don't inherently define subclasses based on drop probability; they map directly to IP Precedence values.