Nikon D6 Reference Manual page 211

A
3D-Tracking
When [3D-tracking] is selected for AF-area mode, the camera stores the
colors in the area surrounding the focus point at the moment the shutter-
release button is pressed halfway. Consequently 3D-tracking may not
produce the desired results with subjects that are similar in color to the
background or that occupy a very small area of the frame.
A
Quick Focus-Point Selection
Choosing [27 points] or [15 points] for Custom Setting a4 [Focus points
used] reduces the number of focus points available via the multi selector,
speeding focus-point selection.
A
Auto-Area AF Starting Point
Selecting [Enable] for Custom Setting a12 [Auto-area AF starting point]
lets you choose the starting focus point when AF-C is selected for AF
mode. After you focus on the subject in the selected point, the camera will
resume normal auto-area AF. This feature is recommended when your
subject's position in the frame can be predicted with some degree of
confidence.
A
AF-S/AF-I Teleconverters
If [3D-tracking] or [Auto-area AF] is selected for AF-area mode when an
AF-S/AF-I teleconverter is used, [Single-point AF] will automatically be
selected at combined apertures slower than f/5.6.
Choosing How the Camera Focuses (Focus)
159