Hi Leonard, I'll just jump in also,

> What does "shrink surface" mean?

After you trim or do booleans, you can have an "underlying surface" that is larger than the trim curves on the surface.

There is some description and illustration of how trim curves and underlying surfaces work here:
http://moi3d.com/wiki/FAQ#Q:_Why_does_show_points_work_for_some_objects_but_not_others.3F

It is possible to set up a keyboard shortcut for a command named "ShrinkTrimmedSrf" which when applied will shrink the underlying surfaces of an object so that they are only as large as the trim curves instead of possibly having excess area.

That can actually reduce the 3DM model size if you have a lot of pieces that are booleaned out of larger initial surfaces. But usually it won't do that much for things that are heavy with fillets though, and it won't have any effect on the polygon generation to OBJ format for example.


> And, why do we have to "separate the polysurface" and how is that done?

That means selecting a solid and running Edit/Separate on it to break it into individual surfaces that are not joined to each other anymore.

You typically have to do that if you want to turn on control points for objects that have been booleaned, some more info on that in the above link.

- Michael