Portals under zdoom

Hello, I was always wandering what the things "upper sector" and "lower sector" could do.
These things are placed under the "portal" section wich can be find in zdoom(doom in hexen format) or skulltag(doom in hexen format).
So, I wander if anyone could clear me up on this?
Thanx in advance.

http://zdoom.org/wiki/Stacked_sectors

Oh ok thanx alot ^^