How do we define the shading impact of adjacent buildings on the building of interest in EnergyPlus/OpenStudio?

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If you choose to simulate detached pavilions (as a single facility/building, as discussed here), then make sure to select FullExteriorWithReflections. You should nonetheless do a few sanity check runs to ensure mutual self-shading & reflections (i.e. between pavilions) are simulated as expected.

If instead simulating each building independently (10 buildings == 10 IDFs), then the other buildings would need to be modelled as shading objects (see here and/or here), and yes also select FullExteriorWithReflections.