The Infinite Horizons system consists of a set of "base rules", which are all that are really required to play. These rules include no built-in assumptions about the setting or genre that will be used, the type of campaign, or the races of the characters.
Rule supplements will be available which detail different settings (ie. fantasy, science fiction, espionage, etc.). These supplements will not repeat anything from the base rules, so you will only be paying for 100% new material.
Each supplement is also guaranteed to be completely compatible with all other supplements, so that portions of different supplements can be mixed and matched to produce the game you want to play, and characters made for one setting can be integrated into another without modification (their chosen skill sets may be less than ideal, but that's to be expected under such circumstances).