What does the shading surface type "Space" do? I do not see any information in the EnergyPlus documentation. It appears to assign a shading surface group to a space instead of to the building or site (the other options). Does this mean that shadowing calculations are only run on the associated space only? 
After some tinkering, it appears to run shadowing calculations on all of the spaces, regardless if the shading surface group is assigned to a particular space.
You are correct, you could say that the choose between using space (which becomes zone) shading and building shading is whichever is most convenient for modeling, however I would generally put window overhangs in a space and a parapet or covered entry as building, but it doesn't mater so long as it is in the right location. The choice between building at site shading is much more important. While building shading rotates with your building site does not. Think of site shading as adjacent context that you don't want to alter. When using non-0 rotation and site shading carefully chose origin.
Thank you @David Goldwasser
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