Failure domain
Beginner
How much stops working when one part breaks.
Novice
The set of things that go down together when one component fails. A failed pluggable module takes down one port; a failed co-packaged engine can take down many ports, or a whole switch until it is repaired.
Expert
Scope of impact × time to repair. Pluggables have a small domain and minutes-scale repair; CPO enlarges the domain and lengthens repair to a board or chassis swap, so it needs much lower FIT rates, redundancy or graceful lane degradation.