Working with mesh edges marked for rendering using the Freestyle render engine

To create line-based renderings, such as blueprints or 3D to 2D stylizations, an additional "Freestyle" render engine is usually used with the main render engine. Edges that should be clearly outlined in the final render must be marked as "freestyle" so that the Freestyle render engine can process them.

Read more: https://b3d.interplanety.org/en/working-with-mesh-edges-marked-for-rendering-using-the-freestyle-render-engine/