This option can be used when you want to find the intersection between two planes, which is needed when locating roof valleys and determining where a sloping plane passes through a given height (for example, when modelling dormer windows).
First of all, create and apply the planes, and then set a 2D plan view to receive input. Use the Select Space tool to select one of the sloped surfaces, and on the Tools menu, click Intersect Planes. Click on the down-arrow for "Select 1st Plane" and "Select 2nd Plane", and then choose the relevant two planes in order to create an intersection line (which is actually a <null> line) as a projection on the 2D plan of where the two planes would meet.
To begin with, you must prepare the geometry by creating the spaces.
Now create and apply the planes, as normal. It doesn't matter whether you use the "by points", "by inclination", or "by offset" method. As you can see, at this point in time the planes do not yet intersect. The next step is to determine the intersection lines, delineating where the planes would intersect.
With respect to the main oblong area, the north and south spaces were used to create the "North" and "South" planes, respectively. There is also an east wing and a west wing. With respect to the east wing, the east and west spaces were chosen to define the "E Wing East" and "E Wing West" planes. The same naming convention was used for the planes on the west wing.
So, you need to select two planes on the Create Intersection Line dialog box. In reference to the screenshot, the southerly space was highlighted with the "Select Space" tool and then the Intersect Planes option was chosen on the Tools menu. On the "Select 1st Plane" drop-down list, the South plane was selected. On the other drop-down list, the "W Wing East" plane" was selected. The red <null> line indicates where the two planes would intersect - in other words it delineates the projection on the plan of where the two planes would meet.
Repeat the process outlined in Step 3 until all of the intersection lines have been determined.
Delete the line segments which are not needed and extend any walls to meet with the intersection lines.
Check the 3D view for accuracy.