The difference between .9999 Gold and .999 Gold bar

An ingot is simply a bar of gold; it may have some standard specification as to size or weight, a .9999 is refined gold (highly refined---that means very little impurities like silver, iron and copper are in the total mix)...it's really pure, only a few mints in the world make such Gold bullion/bars with that purity. (Canada is one of them : The Royal Canadian Mint's Gold Maple Leaf coins are