Ah, it isn't fully populated yet for the public. My mistake. (Going through technical review at this time.) 

 

Exactly.

Do you know the answer to my question? I've seen it asked a couple of times in the forums but never seen it answered. 

Effectively more refinement per iteration, with an increase in render time. And this is not the first time I have answered that. :)  And I have never seen a need to go with more than 2, unless it is a very, very rare case.