When Hidden Line is selected, you can specify how ducts and pipes that cross each other (in separate planes) are presented in your project. In two-line drawings, crossing duct and pipe segments are shown such that the lines representing the segment in the furthest plane are shown in a different style to indicate that they are hidden by, and not connected to the segment in the foreground as shown. Hidden Line parameters are only applied when the Hidden Line is selected as the visual style.