Program mode is Auto mode. When in Program mode, you can change the shutter speed and/or aperture to achieve the desired effect. This is the Flexible Program feature - refer to page 75 of the D600 users manual.
As for Program Mode, many people use it. I use "P" mode on my D3S when using my Speedlights outdoors for fill flash. I set the mode to "P" and the Speedlight to TTL-BL mode and it provides the proper amount of flash to balance the ambient light.
Hope this helps......JL
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