Customization
Servers, particularly those of PC games, can generally be customized in ways that still allow unaltered clients to connect to them. These customizations can include tweaks to built-in game settings, content that is downloaded by clients when they join the game, and new code which changes the way that the server behaves.
While server customization is popular with server administrators and players, it can be at odds with the desire of developers and other players for the game to be experienced as intended. It can also aggravate players by enabling abusive administrators to lie about what their server offers.
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