These used to be known as Achievement tests and NOT to be confused with the SAT or ACT. Only about three dozen of the nation’s most selective schools require them, including Ivy League and other private colleges like Williams, Tufts and any of the University of California schools.
Tests are offered in sixteen subjects. Each lasts an hour up to three can be taken on a particular date and it is not possible to take the SAT and a Subject Test on the same day.
Requirements aside, a good Subject Test score can be helpful at virtually any college. Since the subject tests measure knowledge from school classes, it generally makes sense to take them in June at the end of a school year. Chemistry is a common choice for tenth grades though most applicants wait until spring of the eleventh grade before starting subject tests, which to me makes little sense.