About this detail of the Tiger
The instructions tell you to attach all of the road wheels to the tank, including the outermost ones (part E9). After that, in Step 5, you are told to attach the tracks to the wheels.
Then, in Step 6, when the wheels are firmly locked in and tracks are attached to them, Trumpeter introduce Part E3. They state that it's an "option" and that you can attach it to the wheel, or omit it.
But that's not how the Tiger's wheels work. The front outer wheel was often omitted (usually to prevent buildup of mud). When an outer wheel is removed, a hub is exposed. This is what Part E3 represents.
Part E3 is not an "add on" for a wheel. It is the alternative to a wheel.