It's described in the user manuals, also discussed in threads here.
Bottom line: for determining Avisynth Crop() values, mod 2 is fine. For DGSource() cropping, currently nVidia supports mod 2 only for the right and bottom borders. If you try to select mod 2 for the top and left borders and export that to your script, you will get a warning popup. If you must have mod 2 on the top or left, then use Avisynth Crop().
Support forum for DGDecNV