In Blender’s Freestyle, a line set filters which edges are drawn using criteria like visibility, face marks

http://historieblog.dk/index.php?title=Line_Set_Support_and_Fastening_Tips_for_Secure_Installation

In Blender’s Freestyle, a line set filters which edges are drawn using criteria like visibility, face marks, and material boundaries, allowing separate stroke styles for silhouettes and creases.