But to your question, the way it is done is that we've got a primary disc, which has 300 cars on it, which is a ton of cars -- more than Forza Motorsport 2.
The second disc contains three environments, and another 100-plus cars. What you do is actually install all the content from disc two as if it was DLC and then it automatically integrates into your game. So you can play off disc one without installing it, without ripping to the hard drive, and all the disc two content you have installed (similar to DLC) gets put into the career as AI opponents and shows in your career mode, as different races and the tracks change out. You can make the game more deep, if you like, by adding that content.