PCF

The PCF is the rulebook for how subscribers are treated. It hands the SMF the policy that shapes a PDU session — which QoS to apply, what charging model, any gating or traffic-steering rules — and gives the AMF access-and-mobility policy such as allowed slices and RFSP. It pulls subscriber policy data from the UDR and can take real-time input from Application Functions to react to what apps actually need.

A concrete example: a video app's AF signals (through the NEF, or directly for trusted AFs) that a particular flow needs better treatment; the PCF turns that into a PCC rule, the SMF installs the matching QoS flow, and the UPF enforces it on the packets. It's the descendant of 4G's PCRF, modernised for the SBA. When QoS isn't behaving as expected on a session, the PCF's policy decisions are usually the first place to look.