Black Sites have 4 Nodes. They enhance the Defense Bonus of the Units stationed at the Black Site. Each Node increases the Defense Bonus of a specific Unit class:
Node I provides a Defensive Bonus to Light Infantry Units
Node II provides a Defensive Bonus to Mechanized Infantry Units
Node III provides a Defensive Bonus to Armor Units
Node IV provides a Defensive Bonus to Combat Aviation Units
The more Nodes are upgraded, the more powerful the Defense Bonus they provide becomes.
Nodes are destroyed when Black Sites are captured by other Alliances.