If you mark an inner class as static then it becomes much closer to a normal class.

class Car {
    static class Speedometer {

    }
}

You can make instances of it directly without an instance of the outer class.

Car.Speedometer speedometer = new Car.Speedometer();

And it cannot access fields of the instance it was made in, because it was not made in an instance.

class Car {
    int speed; // Speedometer can't magically get this anymore

    static class Speedometer {

    }
}

I would wager that this is the most common kind of inner class to see in real code, despite requiring more words to define¹.