In 3D modeling, a line set defines which edges are rendered as strokes in a non-photorealistic style

https://www.it-core.eu/wiki/index.php?title=HVAC_Line_Set_Routing_for_Tight_Spaces_and_Difficult_Layouts

In 3D modeling, a line set defines which edges are rendered as strokes in a non-photorealistic style, controlling visibility, crease emphasis, silhouette extraction, and intersection highlights for stylized visuals.