Canon EOS 6D Instruction Manual page 143

Differences Between <d> and <A> Modes
In the <A> mode, many functions such the AF operation and metering
mode are set automatically to prevent spoiled shots. The functions you can
set are limited. With <d> mode, only the shutter speed and aperture are set
automatically. You can freely set the AF operation, metering mode, and
other functions (p.342).
About Program Shift
In the Program AE mode, you can freely change the shutter speed and
aperture combination (Program) set automatically by the camera while
maintaining the same exposure. This is called Program shift.
To shift the program, press the shutter button halfway, then turn the
<6> dial until the desired shutter speed or aperture is displayed.
Program shift is canceled automatically after the picture is taken.
Program shift cannot be used with flash.
If the "30"" shutter speed and the lower f/number
blink, it indicates underexposure.
Increase the ISO speed or use flash.
If the "4000" shutter speed and the higher f/number
blink, it indicates overexposure.
Lower the ISO speed or use an ND filter (sold
separately) to reduce the amount of light entering the
lens.
d: Program AE
143