Many objects consist of several surfaces, some of which are trimmed. You can specify such objects using regular surfaces, plus a set of trim boundaries for each surface, so that when the object is drawn, the trimming is performed automatically.
A trim boundary, or a set of trim boundaries, on a surface define the trim region of a surface.
On the surfaces shown, two trim boundaries exist: one on the cylinder 
and the other on the plane 
.
These surfaces can be trimmed back to give the following:
A trim boundary consists of pcurves and surface edges joined together to form a closed region of a surface.
Trim boundaries are created automatically when:
- creating fillet surfaces.
- creating draft surfaces.
- limiting surfaces using Edit tab > Limit panel > Selection and Point.