Difference Between vbs and qfl files in QTP?

QTP can use3 different types of function library extensions: .qfl (not .qfi), .vbs and .txt.qfl is local to QTP and can only be called and used by QTP. .vbs and .txt on the other hand can be used by QTP and by any application other than QTP. So, what I am trying to say is that, you can open notepad and write a function and associate it with your test as a .txt file. Either way, all three can be used as QTP function libraries which contain a function that you can use in your test. Another way to use functions is to directly type them in your respective action.