public List<SubtypeFeatureLayerCreationParams> SubtypeLayers {get; set;}

{
          var subtypeGroupLayerCreateParam = new SubtypeGroupLayerCreationParams
            (new Uri(@"c:\data\SubtypeAndDomain.gdb\Fittings"));

          // Define Subtype layers
          var rendererDefn1 = new UniqueValueRendererDefinition(new List<string> { "type" });
          var renderDefn2 = new SimpleRendererDefinition()
          {
              SymbolTemplate = SymbolFactory.Instance.ConstructPointSymbol(
                    CIMColor.CreateRGBColor(255, 0, 0), 8, SimpleMarkerStyle.Hexagon).MakeSymbolReference()
          };
          subtypeGroupLayerCreateParam.SubtypeLayers = new List<SubtypeFeatureLayerCreationParams>()
{
      //define first subtype layer with unique value renderer
      new SubtypeFeatureLayerCreationParams(new UniqueValueRendererDefinition(new List<string> { "type" }), 1),

      //define second subtype layer with simple symbol renderer
      new SubtypeFeatureLayerCreationParams(renderDefn2, 2)
};

          // Define additional parameters
          subtypeGroupLayerCreateParam.DefinitionQuery = new DefinitionQuery(whereClause: "Enabled = 1", name: "IsActive");
          subtypeGroupLayerCreateParam.IsVisible = true;
          subtypeGroupLayerCreateParam.MinimumScale = 50000;
          //Note: Needs QueuedTask to run
          SubtypeGroupLayer subtypeGroupLayer2 = LayerFactory.Instance.CreateLayer<SubtypeGroupLayer>(
                        subtypeGroupLayerCreateParam, MapView.Active.Map);
      }