The slider controls the number of camera angles that it uses to look for parts. So if you have a lower number on the slider, you have less camera angle positions and are more likely to have part considered invisible. The higher the slider, the more camera angle iterations happening and you are less likely to have parts hidden. Note: The higher the number, the longer the process will take since it has to perform more iterations. Once and a while you might find some green highlights left on the screen from selections previously made and they will not go away. To remove them, go to the Tools ribbon bar and click Recreate Display, that should refresh the graphics and remove the extra highlights.