I'm looking at Ghoul Fever and it says that, but a few lines up it says the save is Fort DC 13, so I'm not sure what they mean by the charisma based part.
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The note is telling you that the Save DC of the disease is based, in part, on the Ghoul's Charisma modifier. This is only important if the Ghoul is (de-)buffed in some way, or if you are creating a modified version of the Ghoul with different stats. It is not normally relevant information.