difference between beanscope and pageflow scope?

It all depends on the kind of functionality that you have implemented in the backing bean. For example, if you want certain backing bean variables to persist from one request to another, then requestScope won't do - the backing bean will be created/destroyed for each user request. As you probably know, the scope determines the lifecycle of the bean, i.e. when the bean is created and when it is destroyed.

Now, backingBeanScope is used exclusively for page fragments (regions).

In our project we are using pageFlowScope. In this case, the backing bean persists for the duration of the page (i.e. mulitple requests for the same page).