Nested classes inherit their visibility from the parent class and are often used to implement functionality local to the class it is nested in. These nested classes can't have a higher visibility than their parent. However, the visibility can be further restricted by using a private modifier for instance. In internal classes the explicit public modifier for nested classes is misleading and thus unnecessary, because the nested class still has an internal visibility.

Noncompliant Code

internal class Outer {
    // explicit public modifier still results in an internal nested class
    public class Nested
}

Compliant Code

internal class Outer {
    class Nested1
    internal class Nested2
}