I have a question. I read code in qSlicerSegmentEditorScissorsEffect.cxx so. What is the idea of it?
The scissors effect allows deleting (or filling) a region intuitively by marking it with the mouse in a 2D or 3D view.
Do you have any documents about it?
See the Slicer user manual for some information: Segment editor — 3D Slicer documentation
There are also many tutorials on YouTube.