Nikon D300 Complete Manual page 526

Recommendations:
1. Make your choice for number of points based upon your
subject, how much of the subject you need to keep focus
on, and how the subject is moving. If you were framing a
person and animal fully (they occupy the entire frame)
and trying to keep focus on the face (eyes), you'd use a
narrow pattern (9 points). If you have a single person or
animal relatively small in the frame but moving a lot
within the frame, you'd use a wide pattern (51 points).
Basically, you're controlling how far from the original
focus selection point you'll allow the camera to follow the
subject. When you have multiple subjects in the scene, it
becomes imperative that you keep the width of the area
you select small enough to only stay on the subject you
want to use for focus. For example, if you're shooting a
team sport like football, using 9 points allows you to
keep focus on one player while using 51 points might let
the focus system bounce from player to player. Overall, I
suggest you error on the "too tight" side rather than the
"too loose" side.
2. The other critical aspect of this function is the difference
that 3D makes on the 51 point options. Here you're
choosing whether or not to keep the focusing solely
chosen by the autofocus sensors, or whether you want to
allow the color detection of the metering sensor to take
part as well (3D-tracking).
Why is color important? Because it allows the camera to
make decisions about the subject even when it moves off
the autofocus sensor area! Let's take a simple example.
You've got a person dressed all in black against a white
background dancing around in the frame. The autofocus
system can only see contrast, so it's fine if locked onto the
face but not so fine if all it sees is the black clothing.
So consider the situation where the person's face
temporarily moves out of the area that has autofocus
sensors. What happens? If you're lucky, the camera will
Thom Hogan's Complete Guide to the Nikon D300
V1.02
Page 526