2012

 When trimming a surface or region with a 3D curve in an
angled, parallel or perspective view (for example, the default
perspective view), the cutting geometry is projected onto
the surface in a direction parallel to the Z direction of the
current UCS.
Projects the geometry based on the current view.View
Projects the geometry in the +Z and -Z axis of the current UCS.UCS
The surface will only be trimmed if the cutting curve lies on the
surface.
None
See also:
Trim and Untrim Surfaces
SURFUNTRIM
Replaces surface areas removed by the SURFTRIM command.
Access Methods
Button
Toolbar: Modeling tool set  Surfaces - Edit tool group (expanded) 
Surface Untrim
Menu: Modify  Surface Editing  Untrim
Summary
If the trimmed edge is dependent on another surface edge that has also been
trimmed, you may not be able to fully restore the trimmed area.
1026 | Chapter 2 Commands