They are suing them for 980 million yen, which is about $9.43 million (USD). This lawsuit apparently goes back to 2002 when Capcom filed for a patent that lets a user acquire new content by combininb an existing game with another piece of software. The exact details on this patent or how Koei Tecmo is infringing on it was not revealed. However, it seems to focus on how games like Samurai Warriors: Xtreme Legends used expansion discs in conjunction with the original game to unlock new content for the base game.
There is apparently another patent in the lawsuit that Capcom says "greatly contributed to the sales" of various Koei Tecmo games. This feature vibrated the controller when an enemy was near the player. Just from total sales profit of 9.73 billion yen, Capcom is looking to earn royalty fees in the 5-10% range from Koei Tecmo.
They're also demanding that many of those games are no longer sold ever again.
So, correct me if I'm wrong here, but didn't SEGA do this "expansion" thing ages ago?
(via Siliconera)