This all has to do with the evaporative emissions systems. Basically, OEMs are responsible for not only tailpipe emissions but all emissions (see PCV system) (this includes emissions from the plastics used (new car smell)). Pretty much anything that turns on the check engine light has to do with emissions. The evap system vents pressurized fumes from the gas tank into the engine to get burned instead of venting them into the atmosphere.
My wife's Century has that code set because the filler neck is rusted and leaks under pressure. It's a PITA to replace the neck and doesn't really hurt the engine, so I keep putting off replacing it.
