Upgrade Nodes are tokens used to pay for Overmind Player upgrades to Alien Units, and the token is then placed on the Alien Upgrade Sheet to mark which upgrade it paid for.
Alien Upgrade Sheet
The Alien Upgrade Sheet keeps track of which types of Alien Units that have received upgrades. Upgrade Nodes are placed on the Alien Upgrade Sheet to mark which upgrades have been made.
There are different upgrade paths available for different Alien Units, but for each Alien Unit type, the Overmind Player may only choose one upgrade path. Once an Upgrade Node has been placed on one upgrade path, the other upgrade paths for that Alien Unit type may no longer be chosen for upgrades.
Upgrades to an Alien Unit type applies to all units of that type.
The Overmind Player may only make upgrades during the Resolution Phase, not during an ongoing Mission.