I think “Liberate” is referring to the game term where a country reclaims the territory of an ally from an enemy power (and removing their Control Marker).
i.e. At the start of the game, Germany “owns” West Russia. When USSR takes West Russia R1, they’re “taking control” of the territory (putting a Soviet Control Marker on the territory).
If Germany counterattacks and takes West Russia on G1, they’re “liberating” the territory (removing the Soviet Control Marker from the territory).
So, by similar logic, when Zombies are the only units left in a territory, they “Take Control” of the territory (put a Zombie Control Marker on the territory). Then, later on, when your forces survive combat against the Zombies, you “Liberate” the territory from the Zombies (remove the Zombie Control Marker from the territory).
I imagine that, in the case where your troops are removing Zombies from a territory originally owned by an enemy, you “liberate” the territory from the Zombies before “taking control” of it yourself.
Example:
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West Russia is naturally a German Territory.
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Zombies take control of West Russia by some means. Zombies “Take Control” of the territory by placing the Zombie Control Marker on it.
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On Russia’s turn, the Soviets move a stack into West Russia. They easily survive the Zombies.
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At this moment, the Zombie Control Marker is removed from West Russia. Thus, the Soviets have “liberated the Z controlled territory”, and receive any and all applicable bonuses.
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After the “liberation” of West Russia from the Zombies by the Soviets, the Russians “attack” the German Territory “West Russia”.
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As Germany has no units present in West Russia, the Soviets win without a fight, and “Take Control” of the territory by placing the Russian Control Marker.