I checked the Oracle's official web site about OCP 6 exam. The objective 1 has inner and static inner class involved.
It is in Chapter 8 of KB's study guide. According to KB, inner class is not in the objective, but it will appear everywhere in the questions. Test takers need to recognize it.
But it seems like inner class becomes a topic for the exam. So, I think I need to know how inner class is intantiated, inner class can access private member of the outer class, how inner class access outer class's methods.....
Develop code that declares classes (including abstract and all forms of nested classes), interfaces, and enums, and includes the appropriate use of package and import statements (including static imports).
I think it's clear that inner classes are required for the exam. Where does it say that they are not an objetive?
This is the intro for KB chapter 8:
Inner classes (including static nested classes) appear throughout the exam. Although there are no official exam objetives specifically about inner classes, Objetive 1.1 includes inner (a.k.a. nested) classes. More important, the code used to represent questions on virtually any topic on the exam can involve inner classes.
There is no specific objetive about inner classes, but there is one wider regarding all kind of classes with all it forms.