The defining line determines where the section cuts the objects it contains. The material component which controls the display of the Defining Line is the Linework component. The defining line is a display component of both 2D and 3D section objects.
For example, if you cut a brick wall that contains a window, you can specify one uniform color for the defining line, or use different colors where the section line cuts the wall and where it cuts the window. If you want to use a uniform color for the Defining Line component, assign a color to it and do not use the properties of the material. If you want the color of the defining line to be influenced by the objects in the section, use the material properties. In the case of a section through a brick wall with a window, you might use a red defining line where the wall is cut and a blue defining line where the window is cut.
Specifying cut line colors for different materials