In many kinds of things, it really is just fine to use interchangeably. Any recipe should work with some kind of fat, be that butter or shortening. The main difference comes out when you are looking at pastries like pie crusts. Butter tends to be somewhat more flavourful, but you get a lighter, flakier crust with shortening. In things like cookies, I'll often use butter even if it calls for shortening, but many US recipes for different pastries, I prefer shortening.
Oh, and they are right, shortening and Flora White are both vegetable fats and vegetarian happy.