Base of Documents ?

Hi,

I don't know what "Base of Documents" means (right click on a folder in
the left area).
I supposed it was possible to define a subfolder in my folder's tree to
be the root of the uploads, and that this menu would do it. But Quanta
allways sends the files from the root of my tree.

Can someone explain me what this "Base of Documents" means ? I didn't
find anything in the doc

Thanks
Pierre