The D800 utilizes the Nikon Multi-CAM 3500FX autofocus sensor module with TTL phase detection.
Phase-detect autofocus sensors are essentially tiny rangefinders. The light from then lens is split into two paths, and the two paths are projected onto small linear sensors. The difference between the pattern of light and dark is analyzed, and the amount of front or back focus instantly calculated and the camera then translates that into lens focus movement so the lens then autofocuses to be in focus, as long as the lens is at a distance where focus can be achieved.
Actually, all AF sensors are linear sensors (line), but at some sensor locations, the camera manufacturers, like Nikon, pair the sensors and orient them 90° from each other giving us the cross-sensor. The pair work together to determine focus.
The cross-sensor is superior to the regular linear sensor because it can analyze light patterns with both vertical and horizontal patterns while the linear sensor, according to its orientation can only analyze light patterns in either a vertical or horizontal pattern. Focusing via a cross-sensor focus point will be both faster and more accurate than via a linear focus point.
In the D800 and the D4 (both have the same autofocus system) of the 51 AF sensors the central core of 15 sensors, 5 vertically by 3 across, (camera in landscape orientation) are all cross-sensors, and the other 36 sensors are all linear sensors.
In our discussion in this thread, we are discussing how the base or maximum aperture of a lens effects which sensors are available to the photographer, and which cross-sensors retain their cross analyzation ability. It turns out that when the lens is f/5.6 or faster, all the cross-sensors are in play as cross-sensors. This would mean that for all Nikon lenses currently being sold, since all are always f/5.6 or faster, all the cross-sensors can be used and all remain cross-sensors, however, if you use a teleconverter on a lens, and it raises the base or maximum aperture above f/5.6 then fewer cross-sensors are available for use, and some may then only be used as linear sensors.
This is important, as an understanding of the differences and changes according to the base or maximum aperture will help the photographer better utilize the camera's autofocus system.