public GraphicsBuffer indexBuffer { readonly get; set; }

You can use a common index buffer as a pool to define multiple geometries used by ray tracing instances that have different properties (for example different materials). Use RayTracingMultiGeometryInstanceConfig.subGeometries array to configure index ranges for each geometry.

Specify for the GraphicsBuffer.Target parameter when you create the index buffer from scripts. If the buffer is also used in rasterization, then add the flag.