This will probably make no sense, but I need to remember it and so I'm going to write it here for myself :P
smoothing split = uv split w/ padding on the bake
uv split = doesn't need a smoothing split
(This is for normal mapping for games. I've realized that UVing for game models goes against everything you thought you knew about UVing :P)
Going to do some baking this afternoon now on my bag!