Code was added to allow an empire to nationalize only certain centers on a planet. Previously if you nationalized a planet all centers became yours. However in some instances nationalizing a planet meant also nationalizing your allies centers along with the one you actually wanted to nationalize. It was possibly an intentional design of the game to make nationalizing a tougher decision. Do not know for sure but it seems a little one sided when the owner of the center on your planet can shut it down but you can't nationalize the center. So now an additional code can be typed into the special number slot to selectively nationalize a center. Here are the codes.
1 = Mining Centers
2 = Ag Center
3 = Trade Center
If you leave the special number slot blank the code works as before, nationalizing all centers on the planet.