The 2016 World Championship winning Double Down tire has been designed to fit between the Impact and the Zipp when you have a slightly dusty surface, but you need better wear characteristics due to an abrasive surface or a surface that is polished and creates lots of wheel spin.  The pin size and spacing has been adjusted to give more room for dust to clear the tread pattern yet still have the pin density across the carcass to give it great wear characteristics.