Doc:2.6/Manual/Render/Freestyle/Parameter Editor
Parameter Editor
The Freestyle Parameter Editor is a user-friendly interface to define and control line sets and line styles.
Line sets control which of the edges detected by Freestyle will actually be used (rendered).
Line styles control how the selected edges are rendered.
A view map (hence a render layer) can have multiple line sets, and each line set is linked to one line style.
