Even though billed as a flour, tapioca is in fact considered a starch. It is starch extracted from the cassava plant. Lower in carbohydrates than potato (which is already low in carbs), it has a mild aftertaste and therefore is best used in combination with flours or ingredients that have a robust flavor. That is to say, you can get away using more tapioca in a chocolate dessert since the chocolate will overpower any lingering aftertaste the tapioca may impart. In a more delicate cake such as a lemon cake, it’s important not to overdo the tapioca since the subtlety of the lemon would be no match for the slight metallic notes of the starch.