I notice there is some confusion from time to time between the Soft, Smooth and Hidden property of edges in SketchUp. Soft and Smooth is often mixed up and less people know the difference between these properties than soft/smooth vs hidden.
They affect not only the edge itself, but also the visual appearance and selection method of the connected faces.
Hidden: Affects only the visibility of the edge itself.
Soft: Affects the visibility of the edge and joins the connected faces as a Surface – if you click on one face to select it, it will select all neighbouring faces connected by a soft edge.
Smooth: Affects the shading of the connected faces, blending it to give the illusion of a smooth surface. Notice that it does not affect the appearance of the edge in any way.
Look closely at how the different properties affect the visual appearance of the edge and its connected faces in their own different way in addition to how it affect the Select tool.
The reason most people never observe the difference is because the smoothing tools in SketchUp set both properties at the same time. As a Ruby API developer you want to ensure you do the same if you intend to replicate the behaviour of SketchUp’s smoothing.
Trivia: Soft and Smooth edges where added in SketchUp 3, before that you could only hide them.