What is on the SAT Subject test?

What is on the SAT Subject test?
Do I take a normal SAT section and then choose one to three subjects I want to take, or is it just the subjects I want to take for the entire test?

Only the highest ranking Universities require SAT II Subject Tests, and they only require 2 or 3 Subjects. There are 20 different Subjects to select from, and when you go in for your test, you can take up to 3 or 4 in one day. As only the top ranking Universities really require them, only the brilliant students take them, so the Percentiles end up being very skewed. For most of the Subject tests, even a score of 760 out of 800 would only be considered a 70th to 80th percentile. So, unless you know you’ll get at least a 780, it’s really not worth taking an SAT Subject Test as a low Percentile will hurt your chances.

The SAT test that is required for acceptance to University is the SAT Reasoning Test (also known as the SAT I). It is made up of:
Critical Reading
English/and an Essay