Hi Pelle - in this case, the original curve and the trimmed edge are .002 apart - and I was only a little bit careful in moving he corner point. to make the hard corner- the corner point in a case like this only needs to move a small amount. I also extended the edges of the surface (
ExtendSrf) so that the trim curves would be completely within the boundaries of the surface. In any case, I think you’ll find the trimmed result is pretty close to what you were shooting for.
To answer your question about trimming though:
When trimming a surface with a curve in a plan parallel view like the default Top, Front, and Right view, the cutting curve is projected on the surface in the view direction.
When trimming a surface with a planar curve in an angled parallel or a perspective view like the default Perspective view, the cutting curve is projected on the surface in a direction perpendicular to the curve plane.
When trimming a surface with a 3-D curve in an angled parallel or a perspective view, the cutting curve is pulled on the surface by closest points.
Also, possibly helpful for:
Pull commands pay attention to History, so if you Project with History recording on, you can edit the original of the Project operation and the projected result will update.