Hi Raven, you're welcome!

> So what is the difference between a Plane and a square (or rectangular) Surface?

Construct > Planar makes a trimmed plane, you can see the difference if you select each surface and used Edit > Show pts to turn on the surface's control points.

It would actually be ok to make the square with Construct > Planar but if you tried to make the skewed surface with it that would not work for flow because the "underlying surface" of the trimmed plane is not skewed:



- Michael