Both flours are different in ingredients as well as have different significance. Pastry flour has a low protein count as compared to regular flour. Pastry flour has around 8% to 9% protein and if we talk about regular flour it contains 10% to 12%. A low count in protein help baker to make lighter doughs and higher protein will result in the amount of more gluten in the flour which makes denser dough. Both the flours have different use and significance and both the flours are used for making different products. A regular flour consists of a high amount of gluten and thus it is useful in making pizza Dough, Bread, and products that need elasticity. Pastry flour is easy to find and if you are baking something that is not bread then you must go for the pastry flour.
You can also substitute your pastry flour when you run out of it, all you have to do is take 1 cup of normal flour and add 2 tablespoons of corn starch, the mixture will decrease the protein count of the normal flour and you can easily use it to make pastries.