What's the difference between Solo Sum & Sum Select?

I looked in the Nuendo manual, the Artist MC Control User’s Guide, Google and a search here. But I can’t find ANY information on this feature in the MC Control. What’s the difference between Solo Sum & Sum Select? They both appear to do exactly the same thing.

Solo Mode = Multiple Solo Selections
Solo Sum = Single Solo Selection
Sum Select = Single Solo Selection

Why is Sum Select necessary? What does it do that Solo Sum doesn’t?