Hi,
I was just confused and wanted clarification for what exactly SAT IIs are. Are they required or just recommended? Do I take the SAT II(s) with the actual SAT/ACT? When should I start preparing for them - after I take the AP class for the subject or is that not needed?
Specifically for me: If I wanted to go to UCLA and plan on studying biology for pre-med track how many should I take? Which ones should I take? (I will be a high school sophomore in the fall). Thank you :)))
SAT II exams are subject specific and depending upon the college, some require them, some recommend them and some do not consider them in their admission decisions. For the UC’s, they are recommended for some majors at some campuses. UCLA only has recommendations for Engineering applicants and not Biology major applicants however if you do well on the exam(s), they can enhance your application. If you are interested in taking some Subject tests, you should consider Biology E/M or Chemistry and Math 2 which are the common tests for STEM majors. I would take the subject tests after you have taken AP Biology, AP Chemistry or Algebra 2 so the information is fresh. You can go to your local library and find some SAT subject test review books or practice tests are available on the CB website.
You take them separately from the SAT I and/or ACT and you can take up to 3 Subject tests in one sitting.
More information can be found on the College Board website: https://collegereadiness.collegeboard.org/sat-subject-tests/subjects
Exactly! The subject tests are a way to signal an admissions officer that you are a strong student who is an expert in a certain field. It can be a great way to complement your overall story (your personal statement, your major) or even “distract a little” from your SAT performance. Don’t get me wrong: your SAT score will still be looked at first. But if you want to major in Spanish and score 800 on the subject test but only 500 in Math and 700 in English, the AO would not weigh your Math performance as heavily. SAT subject scores are just one more way to stand out (if you perform well).
In terms of necessity: it depends on the schools your want to apply to. I know that some schools (e.g. Georgetown) strongly encourage you to take at least 2. Most schools however state that it is optional. Given the continuously dropping acceptance rates of top schools you are probably a more competitive applicant if you do have subject test scores. Bottom line: You should only take the subject tests if you can score a near perfect score because the students who take these tests are usually experts in the field. If you don’t think you would score too well you can always make up for it with extra curricular or — the most important part: your personal statement. I hope this helps! Let me know if you have further questions and do not hesitated to pm me! 
good luck
SAT Subject Tests have not been officially called SAT II for many years.
The SAT Subject Tests are supposed to cover high school level material, so high school biology and chemistry (not necessarily AP) should be enough for those ones. But math level 2 should be taken after precalculus and trigonometry. Students who will complete algebra 2 in 11th grade are supposed to take math level 1, which covers up to algebra 2.
https://collegereadiness.collegeboard.org/sat-subject-tests/subjects/science/biology-em
https://collegereadiness.collegeboard.org/sat-subject-tests/subjects/science/chemistry
https://collegereadiness.collegeboard.org/sat-subject-tests/subjects/mathematics/mathematics-2
https://collegereadiness.collegeboard.org/sat-subject-tests/subjects/mathematics/mathematics-1
Since you mentioned UCSD in another thread, UCSD’s website says,
If you have your eyes set on UCSD, take an AP science and the corresponding subject test in May or June (after you have taken the course) of your junior year.
https://admissions.ucsd.edu/first-year/application-requirements.html