The term 'nested top-level' class is oxymoronic. The very definition of a nested class is any class that is not a top-level class. In other words, the two a mutually exclusive and do not intersect - not only that, they complete the set of "all classes".
Also from a coding perspective the nested classes are hard to read and maintain. Also very rarely does it get used. If I were you, would not spend too much effort. It is just good to know, how it is done.