Originally, the SAT I was meant to test aptitude and the SAT II was meant to test achievement. That is to say, one tested what you were capable of and the other tested what you knew. But when the College Board moved away from the idea that the SAT I tests innate ability, they framed it as a reasoning test, making the difference between the two less defined.
With the latest changes, the SAT I (now just the SAT) is more focused than ever testing knowledge rather than logic. At this point, I would say that the SAT tests general knowledge and and the Subject Tests assess topical knowledge.
