What I mean is that a player somewhere will build a house and use third party custom content. Perhaps a modified plant or a window. Or maybe they've put custom content on the Sims as clothing that wasn't purchased at the Store. When this happens, that content is in the file that is packaged and uploaded to the Exchange on the official Sims 3 site. Then when someone else downloads it, that custom content comes along with it and becomes part of their game even if they don't want it. Many times this is bad bad bad because the custom items will really mess up the game. I had "nude" clothing get into my game this way more than once. I was not happy, to say the least.
