Hi Breeze,
May I ask which Total War game you are referring to?
M:TW VI (Viking Invasion expansion pack) and S:TW MI (Mongol Invasion expansion pack) both add new units to the main campaign.
In addition VI allows increased modability and unlocks some of the previously unplayable factions. Due to this, M:TW VI is required to play the majority of modifications for Medieval.
R:TW BI (Barbarian Invasion expansion pack) and M2:TW Kingdoms do not make any changes to the main campaign - they both run through a separate .exe file to the original copy, so in theory could be considered separate games.
Hope this helps, good luck, cheers!
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