What is the widest tire for the 27.5+ width rims?
I'll venture a guess that currently there are two lines defining the "plus" range you are talking about.  Anything under 2.75 to 3 inches will likely be defined as a regular MTB tire.  These were often called "fat tires" until Surly popularized the Endomorph, which made the term "fat tire" fall out of favor for regular mountain bike tires.

The original Endomorph was (I believe) a 3.7 inch tire which sort of starts the current range of "fat tires" and goes up.  "Plus" tires are a relatively recent thing that Surly also had a big hand in (look up the Krampus or the Knard) that consists of tires to big to fit the frames of most regular mountain bikes, but still a bit smaller than the current "fat tire" market (3.7 inches and up).  I'd say the current "Plus" range is maybe 2.7 inches to 3.7 inches.

One of the current uses for the plus set that I have seen is running 29'r plus tires and wheels in a 26 inch "fat bike" frame for summer or hard trail conditions.  

Again all of these definitions may change as the market and common tire sizes change, but I believe that's about the current state of things.  When looking at these setups perhaps the most important thing to consider is what size the frame will be required to fit the setup.