In Xcode's **Project Navigator**, I see three types of container/folder icons:
https://i.stack.imgur.com/cztUw.png
So:
- A yellow/golden folder (which I can create by right-clicking my project and going to New Group); and
- A blue folder with squares inside of it; and
- A blue folder with nothing inside of it
I'm wondering what these three "folder types" are, when to use them (and why) and how to create them.