First draft

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Editor/Material/Lit/BaseLitUI.cs

 
         public enum MaterialId
         {
-            LitSSS = 0,
+            LitSSSAndTransmission = 0,
-            LitClearCoat = 3,
-            LitSpecular = 4,
-            LitIridescence = 5,
+            LitIridescence = 3,
+            LitSpecular = 4
         };
 
         public enum HeightmapParametrization
 
             // Set the reference value for the stencil test.
             int stencilRef = (int)StencilLightingUsage.RegularLighting;
-            if ((int)material.GetFloat(kMaterialID) == (int)BaseLitGUI.MaterialId.LitSSS)
+            if ((int)material.GetFloat(kMaterialID) == (int)BaseLitGUI.MaterialId.LitSSSAndTransmission)
             {
                 stencilRef = (int)StencilLightingUsage.SplitLighting;
             }

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/LayeredLit/LayeredLit.shader

         [Enum(Flip, 0, Mirror, 1)] _DoubleSidedNormalMode("Double sided normal mode", Float) = 1
         [HideInInspector] _DoubleSidedConstants("_DoubleSidedConstants", Vector) = (1, 1, -1, 0)
 
-        [Enum(Subsurface Scattering, 0, Standard, 1)] _MaterialID("MaterialId", Int) = 1
+        // For layering, due to combinatorial explosion, we only support SSS/Transmission and Standard. We let other case for the shader graph
+        [Enum(Subsurface Scattering and Transmissison, 0, Standard, 1)] _MaterialID("MaterialId", Int) = 1 // MaterialId.Standard
 
         [Enum(None, 0, Vertex displacement, 1, Pixel displacement, 2)] _DisplacementMode("DisplacementMode", Int) = 0
         [ToggleUI] _DisplacementLockObjectScale("displacement lock object scale", Float) = 1.0

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/LayeredLit/LayeredLitTessellation.shader

         [Enum(Flip, 0, Mirror, 1)] _DoubleSidedNormalMode("Double sided normal mode", Float) = 1
         [HideInInspector] _DoubleSidedConstants("_DoubleSidedConstants", Vector) = (1, 1, -1, 0)
 
-        [Enum(Subsurface Scattering, 0, Standard, 1)] _MaterialID("MaterialId", Int) = 1
+        // For layering, due to combinatorial explosion, we only support SSS/Transmission and Standard. We let other case for the shader graph
+        [Enum(Subsurface Scattering and Transmissison, 0, Standard, 1)] _MaterialID("MaterialId", Int) = 1 // MaterialId.Standard
+
         [Enum(None, 0, Tessellation displacement, 3)] _DisplacementMode("DisplacementMode", Int) = 3
         [ToggleUI] _DisplacementLockObjectScale("displacement lock object scale", Float) = 1.0
         [ToggleUI] _DisplacementLockTilingScale("displacement lock tiling scale", Float) = 1.0

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/Lit/Lit.shader

         [HideInInspector] _UVMappingMask("_UVMappingMask", Color) = (1, 0, 0, 0)
         [Enum(TangentSpace, 0, ObjectSpace, 1)] _NormalMapSpace("NormalMap space", Float) = 0
 
-        [Enum(Subsurface Scattering, 0, Standard, 1, Anisotropy, 2, ClearCoat, 3, Specular Color, 4)] _MaterialID("MaterialId", Int) = 1 // MaterialId.RegularLighting
+        // Following enum should be material feature flags (i.e bitfield), however due to Gbuffer encoding constrain many combination exclude each other
+        // so we use this enum as "material ID" which can be interpreted as preset of bitfield of material feature
+        // The only material feature flag that can be added in all cases is clear coat
+        [Enum(Subsurface Scattering and Transmissison, 0, Standard, 1, Anisotropy, 2, Iridescence, 3, Specular Color, 4)] _MaterialID("MaterialId", Int) = 1 // MaterialId.Standard
 
         [Enum(None, 0, Vertex displacement, 1, Pixel displacement, 2)] _DisplacementMode("DisplacementMode", Int) = 0
         [ToggleUI] _DisplacementLockObjectScale("displacement lock object scale", Float) = 1.0

ScriptableRenderPipeline/HDRenderPipeline/HDRP/Material/Lit/LitTessellation.shader

         [HideInInspector] _UVMappingMask("_UVMappingMask", Color) = (1, 0, 0, 0)
         [Enum(TangentSpace, 0, ObjectSpace, 1)] _NormalMapSpace("NormalMap space", Float) = 0
 
-        [Enum(Subsurface Scattering, 0, Standard, 1, Anisotropy, 2, ClearCoat, 3, Specular Color, 4)] _MaterialID("MaterialId", Int) = 1 // MaterialId.RegularLighting
+        // Following enum should be material feature flags (i.e bitfield), however due to Gbuffer encoding constrain many combination exclude each other
+        // so we use this enum as "material ID" which can be interpreted as preset of bitfield of material feature
+        // The only material feature flag that can be added in all cases is clear coat
+        [Enum(Subsurface Scattering and Transmissison, 0, Standard, 1, Anisotropy, 2, Iridescence, 3, Specular Color, 4)] _MaterialID("MaterialId", Int) = 1 // MaterialId.Standard
+
 
         [Enum(None, 0, Tessellation displacement, 3)] _DisplacementMode("DisplacementMode", Int) = 3
         [ToggleUI] _DisplacementLockObjectScale("displacement lock object scale", Float) = 1.0