occupied area

The line(s) of sight need to be drawn in both plan and section. For each window, you will have two lines. One from each point on the window's exterior opening to the opposite side of the windows interior opening, then continuing into the room to the first opaque object. The space between these two lines that form an X with each other, is the area that qualifies per the plan view. You do the same thing in section view using the exterior and interior openings for the windows from the top to the bottom and bottom to the top. The area that qualifies is the area that qualifies for both the plan and the section. This answer is obviously more clear with images. If you look at page 49 of "Windows and Offices: A Study of Office Worker Performance and the Interior Environment" which is referenced in the credit and included here under Resources, you will see a drawing with a small illustration of an eyeball and the required site lines.