SAT I:

The SAT measures the critical thinking skills you'll need for academic success in college. It assesses how well you analyze and solve problems. SAT scores are used for college admission purposes because the test predicts college success. The SAT I is a three-hour test that measures verbal and mathematical skills. SAT I scores are used for college admission purposes because the test predicts readiness for college work.

SAT II:

The SAT II: Subject Tests are one-hour, primarily multiple-choice tests in specific subjects. The Subject Tests measure knowledge or skills in a particular subject and your ability to apply that knowledge. Many colleges require or recommend one or more of the Subject Tests for admission or placement. The Subject Tests are one-hour, primarily multiple-choice tests in specific subjects. Subject Tests measure knowledge or skills in a particular subject and your ability to apply that knowledge.

Most colleges require the SAT I for admission and many other schools require both the SAT I and SAT II: Subject Tests for admission purposes or placement. Additionally, some colleges require specific Subject Test tests while others allow you to choose which tests you take. It's best to check directly with the school.