Comments.

com.unity.render-pipelines.high-definition/HDRP/Material/Lit/Lit.hlsl

 
     // If refraction is enable we use the transmittanceMask to lerp between current diffuse lighting and refraction value
     // Physically speaking, transmittanceMask should be 1, but for artistic reasons, we let the value vary
+    //
+    // Note we also transfer the refracted light (lighting.indirect.specularTransmitted) into diffuseLighting
+    // since we know it won't be further processed: it is called at the end of the LightLoop(), but doing this
+    // enables opacity to affect it (in ApplyBlendMode()) while the rest of specularLighting escapes it.
 #if HAS_REFRACTION
     diffuseLighting = lerp(diffuseLighting, lighting.indirect.specularTransmitted, bsdfData.transmittanceMask);
 #endif

com.unity.render-pipelines.high-definition/HDRP/Material/MaterialEvaluation.hlsl

 void ApplyAmbientOcclusionFactor(AmbientOcclusionFactor aoFactor, inout BakeLightingData bakeLightingData, inout AggregateLighting lighting)
 {
     // Note: in case of Lit, bakeLightingData.bakeDiffuseLighting contain indirect diffuse + emissive,
-    // so Ambient occlusion is multiply by emissive which is wrong but not a big deal
+    // so Ambient occlusion is multiply by emissive which is wrong but not a big deal.
+    // Also, we have double occlusion for diffuse lighting: 
+    // The baked diffuse lighting part from builtinData.bakeDiffuseLighting = SampleBakedGI() already
+    // had precomputed (aka "FromData") AO applied, and will get double occluded from screen space AO
+    // (this was done to avoid storing AOFromData in the GBuffer, hence why GetScreenSpaceAmbientOcclusion*()
+    // is called with AOFromData = 1.0 in Lit:PostEvaluateBSDF(), hence why we only have SSAO in the aoFactor
+    // here)
     bakeLightingData.bakeDiffuseLighting *= aoFactor.indirectAmbientOcclusion;
     lighting.indirect.specularReflected *= aoFactor.indirectSpecularOcclusion;
     lighting.direct.diffuse *= aoFactor.directAmbientOcclusion;

com.unity.render-pipelines.high-definition/HDRP/Material/StackLit/StackLit.shader

     #pragma shader_feature _USE_UV2
     #pragma shader_feature _USE_UV3
     #pragma shader_feature _USE_TRIPLANAR
-    // ...TODO: for surface gradient framework eg see litdata.hlsl,
-    // but we need it right away for toggle with LayerTexCoord mapping so we might need them
-    // from the Frag input right away. See also ShaderPass/StackLitSharePass.hlsl.
-    #pragma shader_feature _BLENDMODE_PRESERVE_SPECULAR_LIGHTING // easily handled in material.hlsl, so adding this already.
+    #pragma shader_feature _BLENDMODE_PRESERVE_SPECULAR_LIGHTING // handled in material.hlsl
     #pragma shader_feature _ENABLE_FOG_ON_TRANSPARENT
 
     // MaterialFeature are used as shader feature to allow compiler to optimize properly