Nero: In a (default layout) compilation window, why are there two panes at left?

After years of using Nero Express for simple multi-session data discs, I’m trying to get to grips with Nero Burning. I have both 6.6 (full) and 7 (Lite) currently installed.

My extremely dumb question:

In a (default layout) compilation window, why are there two panes at left?

And into which of these two panes does one drag stuff?

Come to that, I’m not even sure why there are two browser panes at right.

Like, why not have just one pane for the source and one for the target?

I’m sure there’s a logical reason but I’m durned if I can think of it. Advice would be much appreciated :o

I suggest you start here: http://www.nero.com/nero6/enu/FAQs_Products_NeroBurningROM.html