Nikon D6 Reference Manual page 236

❚❚ P (Programmed Auto)
In this mode, the camera automatically adjusts shutter speed
and aperture according to a built-in program to ensure optimal
exposure in most situations. You also have the option of
choosing different combinations of shutter speed and aperture
without altering exposure ("flexible program").
A
Flexible Program
Mode P offers "flexible program", in which
although the camera chooses the initial shutter
speed and aperture, the main command dial
can (while the standby timer is on) be rotated
to select different combinations without
altering exposure from the value chosen by the
camera. While flexible program is in effect, a
flexible program indicator (" U ") appears in the viewfinder and top control
panel.
•
To restore default shutter speed and aperture settings, rotate the dial
until the flexible program indicator is no longer displayed. The default
shutter speed and aperture are also restored when you turn the camera
off or select another mode.
184
Exposure-Related Settings (Metering and Exposure Compensation)