Most of the IGC options are pretty self-explanatory - creating 'free' nations that are not otherwise in the game at the beginning (but can surface in revolts anyways), moving COTs, changing control of certain territories (including the Netherlands region, Ingermanland and Bujak), and turning on/off massive unrest events in some of the large eastern european / asian nations. There's a couple ahistorical options such as Vinland and Byzantium. (Historically the Vinland colony failed, and Byzantium fell to the Turks before the game timeframe.)
I believe most of them are explained in a readme file that is included in the IGC download.