Self-rising flours are generally cake or pasty flours that have leavening and salt already formulated into them. Other than that, they’re the same as other flours. I tend not to buy them very much unless I’m certain I’m getting them from a store with high turnover, since the chemical leavening loses its potency over time. The effectiveness of the leavening also falls off very rapidly after the flour is opened (in just a few weeks), so unless you’re a high-volume biscuit or cake baker, you might want to just make your own. To replace a self-rising cake flour, simply add 1 1/2 teaspoons of baking powder and 1/2 teaspoon of salt per cup of cake flour. To replace a self-rising biscuit flour, add the same ingredients to the same volume of pastry flour.