The certified category is reserved for the highest-risk unmanned operations — for example flying over assemblies of people, transporting people, or carrying dangerous goods. This is the level where drone operations are, in terms of risk, comparable to traditional aviation.
The requirements therefore mirror manned aviation most closely: certification of the aircraft itself, certification of the operator, and licensing of the pilot. For an everyday hobby or A1/A3 pilot this category is not relevant in practice, but the exam expects you to know how it differs from the open and specific categories.