public int indexCount { readonly get; set; }

Use -1 to instruct Unity to use all the indices from RayTracingGeometryInstanceConfig.indexBuffer when building the acceleration structure associated with the ray tracing instance. If the value is positive, it must be a multiple of 3 and together with RayTracingGeometryInstanceConfig.indexStart it must define a valid range into RayTracingGeometryInstanceConfig.indexBuffer.