Some call it Group III and some III+. It is different from standard Group III's. It is Shell's XHVI Group III which is made from a slack wax process. Some of the chemist on here can explain it better than I can. I do know this, it is a very good base stock.
Shell Rotella Synthetic and Helix Ultra use the XHVI group III.
When oil is hydrocracked, long chain paraffin slack wax is cracked off. With hydrotreatment, they can break that long chain wax down into short chain hydrocarbons of roughly equal size with high purity and repeatability levels.
It's in many ways similar to a "labratory" synthetic but the source of the hydrocarbons is slack wax rather than natural gas.
Group III+ is just an informal category to designate base oils with a very high viscosity index for the group. For Group III, usually a VI of ~ 140+ (or much higher than what Group III's normally are).
Group III+ base oils are normally made from the hydro-isomerization of wax (like Shell's XHVI), but can also be made by hydrocracking oil refineries.
Yubase 4-plus (4.0 cSt) … VI = 134
Yubase 6-plus (6.0 cSt) … VI = 145
The normal Group III hydrocracked Yubase base oils have a VI of …
4.0 cSt … VI = 122
6.0 cSt … VI = 130
There are some articles that discuss these base oils over at Yubase
There is also a class of Group III+ oils called GTL (Gas to Liquids) base oils. Here, wax is produced from natural gas, and then the wax is again put through a hydro-isomerization process to produce a high quality base oil.