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Downloadable content (DLC) is content built by Paradox Development Studio (PDS) as an extension or add-on to Crusader Kings III. They are modular in nature, which means that a player can choose to play with or without a given DLC by checking them out at the launch menu. An expansion DLC is normally accompanied by a free patch that gives players most of the new content as not to hinder the playability of the game. As such, most DLCs are seen as optional content for players who wish to have a better gaming experience in exchange for some monetary support.
Multiplayer games also benefit from this compatibility, that is to say if the host has a gameplay DLC (expansions and flavor packs) the player does not, the game acts as if the player has it.
The base game is required for any and all DLC relating to the game. While cosmetic/music DLCs can be used with any game version, content DLCs can't be used prior to the version they were released with.