I’m going with the recruit on this one. Someone in S6 might be able to give a more detailed answer but latest threads vs latest replies seems correct to me.The "What's New" link does not have any kind of Time Mark associated with it. This shows you everything that has come in and in the order in which it has, regardless of which sub-forum it belongs to.
"New Posts" does a similar function except that you can use the Mark As Read button to set a time mark so the forum only shows you what is new since that time.