Nikon D6 Reference Manual page 218

A
The Sub-Selector
•
The sub-selector can be used for focus-point
selection in place of the multi selector.
•
Exposure and focus lock while the center of
the sub-selector is pressed.
•
Be careful not to put your fingers in your eye
while using the sub-selector.
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Portrait Orientation
You may find it convenient to use the vertical multi selector when taking
photographs in portrait orientation. See Custom Setting f3 [Custom
controls] > [Multi selector for vertical shooting].
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Using the Sub-Selector and Vertical Multi Selector
You can press the centers of the sub-selector
and vertical multi selector or tilt them as
shown. The controls may not function as
expected when pressed from the side.
166
Choosing How the Camera Focuses (Focus)