Dataflow
At the end of the Lumen calculation, the results of Diffuse Indirect, Rough Specular, and Lumen Reflection are blended together.
Diffuse and Rough Specular are from: Integrate
Reflection is from: Lumen Reflections
Details
The detail calculation is:
Reference Codes
The code related to the image is as follows:
AO Calculation
Engine/Shaders/Private/DiffuseIndirectComposite.usf : GetShadingOcclusion
Rough Specular
Engine/Shaders/Private/DiffuseIndirectComposite.usf : CombineRoughSpecular
Light Accumulator
Engine/Shaders/Private/LightAccumulator.ush : LightAccumulator_AddSplit