The Alien Unit Tracker is located on the World Map, and is used to keep track of how many Build Points the Overmind Player has spent, as a way of checking that the Overmind Player hasn't accidentally chosen Alien Unit for a total Build Point Value larger than they have available for the Mission, but when playing in Narrative Mode when the other functions of the World Map aren't used, it may be more convenient to track this by some other means, like writing it on a piece of paper.
World Map
The "World Map", also known as the "Global Operation Map", is a game board mainly used in Conquest Mode to keep track of which Regions someone has or hasn't taken control over, which also has the Conquest Tracker for both sides.
The World Map also contains the Alien Unit Tracker, which can be used in both Game Modes.