Use the following options to set what happens when you select Surfaces > Trim Tool.
If you want to keep the region you trimmed away, select Keep. To discard the region you trimmed away, select Discard. The default is Keep.
Causes the underlying surface to shrink to just cover the retained regions. This permanently changes the surface geometry and cannot be restored by untrimming, but Undo will still work.
Specifies how precisely the Trim Tool uses the shape of the curve-on-surface when it trims the surface. The default value works well in most cases, but you can use a smaller value to increase precision.
The original surface is retained after the trim is performed. If the surface and the curve-on-surface were created with construction history turned on, you must keep the original geometry.