Preloader and Transition for Dynamic Files

Going through this tut.

http://www.kirupa.com/developer/mx/preloader_transition.htm

All Went well. Works fine. I am trying to understand the code. Read how it was actually done. I get most of it. My question is where does “_root.section” come from. I see no instances with that name on any timeline. Can anyone explain?

Thanks -Blood.