How does EnergyPlus receive shortwave radiation values for the exterior energy balance?

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Hi all,

In the EnergyPlus documentation, exterior short wave radiation explanation is quite vague https://www.energyplus.net/sites/defa...

It says at a point in the description that. "αsol is calculated using procedures presented later in this manual and includes both direct and diffuse incident solar radiation absorbed by the surface face. This is influenced by location, surface facing angle and tilt, surface face material properties, weather conditions, etc."

But never fully discussed in the documentation later on. Is there a source which explain how the shortwave radiation figures are calculated? Thank you in advance.