An aloo dum biryani is made with potatoes, but it has more of a traditional style biryani gravy. The reason it's called an aloo dum biryani is because it uses the dum style of cooking the ingredients, sealed, in a single pot. A dum aloo biryani, on the other hand, uses a dum aloo gravy made in the Kashmiri dum aloo style, so it has very different flavors.