added #pragma multicompile to StackLit and some comments

com.unity.render-pipelines.high-definition/HDRP/Editor/Material/Decal/DecalUI.cs

         }
         protected string[] blendSourceNames = Enum.GetNames(typeof(BlendSource));
 
+        // relies on the order shader passes are declared in decal.shader
         enum BlendMode
         {
             Metal_AO_Smoothness,

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

     {
         Tags{ "RenderPipeline" = "HDRenderPipeline"}
 
+		// c# code relies on the order in which the passes are declared, any change will need to be reflected in DecalUI.cs
+		// enum BlendMode
+		// {
+		//		Metal_AO_Smoothness,
+		//		Metal_Smoothness,
+		//		Metal,
+		//		Smoothness,
+		//		AO
+		// }
+
         Pass
         {
             Name "DBufferProjector_MAOS"  // Name is not used

com.unity.render-pipelines.high-definition/HDRP/Material/Decal/DecalSystem.cs

                     return;
                 int batchIndex = 0;
                 int totalToDraw = m_NumResults;
-                int shaderPass = instance.perChannelMask ? (int)m_Material.GetFloat("_MaskBlendMode") : 0;
+                int shaderPass = instance.perChannelMask ? (int)m_Material.GetFloat("_MaskBlendMode") : 0; // relies on the order shader passes are declared in decal.shader and decalUI.cs
+                // enum BlendMode
+                // {
+                //		Metal_AO_Smoothness,
+                //		Metal_Smoothness,
+                //		Metal,
+                //		Smoothness,
+                //		AO
+                // }
+
                 for (; batchIndex < m_NumResults / kDrawIndexedBatchSize; batchIndex++)
                 {
                     m_PropertyBlock.SetMatrixArray(HDShaderIDs._NormalToWorldID, m_NormalToWorld[batchIndex]);

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

 
 
     #pragma shader_feature _STACKLIT_DEBUG
+
+	// decal 3RT or 4RT toggle
+	#pragma multi_compile _ _DECALS_PER_CHANNEL_MASK
+
     //enable GPU instancing support
     #pragma multi_compile_instancing