So far, this is what I have learned (via trial and error) about the different starch flours used in gluten free flour blends. When a flour mix calls for cornstarch flour, you can safely substitute arrowroot flour (which is my preference). Often there is a mix of arrowroot/cornstarch and tapioca flours in a flour blend, but if there is just one, you can substitute tapioca for arrowroot/cornstarch and vice versa. I cannot find sweet rice flour (another thickener) anywhere locally, but where some recipes have called for them, I have used tapioca flour and it's been just fine. I've also substituted cornstarch for an equal mix of tapioca and potato starch flours. What I've learned NOT to do is substitute potato flour for potato starch!