A rectangular or arched shape for subtracting niches into walls.
If the Arch Height is zero, the top is flat. The Wall Pen draws a heavy line around the back of the niche, to match the weight of the wall's contour.
The side with the center node should go along the edge of the wall. That side is drawn with the object's pen.
The idea is to SEO subtract the object from the wall, as discussed here. You can use a window to cut a niche, but the subtraction gives you better display control.
You would typically place the object on the layer X SEO Show2. This is the layer for SEO operators that show in plan only. (In 3D views, you would see the niche, but not the operator itself.)
You might need to bring the object forward to make sure it masks the wall. The parameter Edge Nudge helps make sure the lighter edge line of the object covers the heavier wall contour line underneath. Turn on True Line Weights to check it, and increase the nudge as needed.
