So, here's the scenario, lets say you have a solid box. If you create a spherical void that cuts the solid, you would expect the result in the second image based on how Revit "typically" works.
But what if what you were looking for was slighting different, what if you wanted the part of the solid that is being subtracted? Normally in the Revit "world" you would say you need to create a void that is on the "outside" rather then just a spehere, to get the result in the third image.What is I told you that you can create the condition with the same spherical void! You might say, what! How! Well, its seems that normals sometimes do matter in Revit.
To achieve the result in the third image its is all about how you draw the arc that creates the sphere. In the first example the arc is drawn counter-clockwise from the "top" of the axis to the "bottom" of the axis. To get the result in the third image the arc must be drawn clockwise from "bottom" to the "top" of the axis. In this case the differing result is because the normal(s) of the surface that make the void are different based on the direction of the arc, which is controlled by how you draw it.
An interesting little "feature" in Revit/Vasari (to be honest I did this in Vasari), your mileage may vary on what you get depending on what you're doing, but it could be handy and it shows that it pays to pay attention to what direction you draw your lines, even in Revit!
