Chapter 3 taught you to read show dcb pfc and show dcb ets. You can look at those outputs and tell whether PFC is enabled and whether ECN is active. That is diagnostic reading -- recognising a healthy state from a broken one.
This chapter goes one layer deeper. It explains what PFC and ECN are actually doing at the wire level, why they are designed the way they are, how they interact with each other, and crucially -- how they fail. By the end, you will not just be able to read the output. You will understand why the output looks the way it does, which is the only foundation from which you can tune and debug these mechanisms under production pressure.
The story starts with a simple question: what actually happens when a 400G port receives more traffic than it can send?