what does exactly BarDockArea refer to?

There are four BarDockAreas used with BarManager. One is docked on each side of the host container control and is where the dockable toolbars and menubars are added into. So the rows of dockable toolbars are contained within a BarDockArea.

So it's a MUST if I want to use dockable toolbars and menubars, right?
btw, the initial size of BarDockArea doesn't make much sense, system will make it suitable for every use case, right?
thx.

Yes it is needed for BarManager and BarManager will resize it appropriately. You should never need to interact with that control type yourself or add it manually since BarManager does that for you.