OK. I must have read the manual many times over but only discovered its true use today! In fact, its embedded among three other area focus mode - Single point, dynamic area and auto area. How different then is this mode compared to the rest in the Nikon D90?
Single point - Used in conjunction with AF-C to enforce camera focus engine to continuously attend to a single point of a stationary or moving object. User can manually override focal point by clicking on "OK" button behind the camera and selecting an appropriate focal point with the four-way direction pad. Camera will ignore all other areas of the composed image.
Dynamic area - The camera will take into consideration the multiple focal points previously identified around the subject and use them to refocus in event the subject should leaves the center frame. Depending on the AF mode used - AF-A/AF-C/AF-S, user may be able to release the shutter door regardless if focus is achieved first.
Auto-area - Fully automatic default that enables the camera to pick multiple focal points with the subject. This mode does not provide user the flexibility to override or specify a desired area to focus on within the composed image.
3D Tracking - Similar to Dynamic area but with the intelligence to track a moving subject real-time within a stationary or panning frames. Good for unpredictable, moving subjects. This is achieved with the use of the built-in 420-pixel RGB sensor to track subjects in motion.