I just checked on line to see if this is the same as a countertenor. Apparently so. I had a teacher, Enico Luk, who specialised in this and what really surprised me is that they are often baritones. Check out this at 19 mins.
https://www.youtube.com/watch?v=vWmPsXk5KWYI thought it would be kind of cool to add it too the range.... but apparently you have to decide either baritone or countertenor. I guess the tenor voice is too powerful to go that way.
The baritones in Il Divo do some incredibly high stuff, but I couldn't find a clip. The tenors are just incredible - period.
Well worth checking out though as stunning live.