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

https://xeon-wiki.win/index.php/The_Complete_Beginner%E2%80%99s_Guide_to_HVAC_Line_Set_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.