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To address the first part of this question, I believe this was added for the simplified geometry route where EnergyPlus was failing to converge and they found that having more mass would improve convergence. These surfaces will have an effect on the calculation, especially if adjacencies are defined, but going into much more detail than this will make the answer overloaded. A short example would be if you had an interior wall with an adjacency then during the simulation the heat transfer between both spaces and the interior partition is computed.

If you are pursuing the detailed geometry approach then a geometry will be constructed, but if using the simplified approach, which EnergyPro uses, then surfaces have no spatial awareness of each other and thus no shading effect.

The Title 24 non-residential ACM defines what does and doesn't need to be defined in the model. 5.5.10 "Heat Transfer between Thermal zones" is suggestive that partitions should be modeled with adjacencies defined, but it isn't that clear.