Hi nekodaze. The 'Section Size' and 'Section Padding' under the Split Frames section (It is beside but you got the point) is a roundabout way to render frames while using reduced memory size.
It is not recommended and I would not recommend it AT ALL. It will create artifacts in fast moving objects, smokes, and similar action scenes. It will create boxes artifacts on your entire video, like for my case, when I try to render 1920x1080 video (23 hours render), I use 450 Section Size and 150 Section Padding under the split frames section, it creates 8 equal sections on my video when it is done rendered
(only seen in high intensity scenes as mentioned before).
If you still don't get it due to my 1st class English, imagine 8 TVs with frames and all, put together to make 1 big screen. You can still see the border even though it is see-thru (like table glass color).