Many selective colleges require or recommend students to take the SAT Subject Exams (read if recommended for a competitive college, treat it as required to keep up with the competition).  This is in addition to taking the SAT I.  Specific subject tests have additional requirements than just the standard Number 2 pencil and photo ID that the ACT or SAT I require.     

If your child is taking a subject test in mathematics, make sure that they bring an approved calculator on test day.  Test centers will not provide them.  The only Subjects Tests for which calculators are allowed are Mathematics Level 1 and Mathematics Level 2. Test takers must put the calculator away when not taking a mathematics test.  A scientific or graphing calculator is necessary for these tests.  The College Board recommends using a graphing calculator rather than a scientific calculator. If your child is taking a language with listening subject test, your child must bring a CD player on test day. The listening section of the test is on CD.  Give yourself time to find a CD player.  Amazon has a wide variety to choose from.  When signing up for a test date, especially with the language tests, make sure that the subject test is offered on that examination date.  Not all subject tests are offered on every test date.  ​ 

Language Tests: Spanish, German, French, Modern Hebrew, Latin, Italian 

Language Tests with Listening: Spanish with Listening. German with Listening. French with Listening. Chinese with Listening. Japanese with Listening. Korean with Listening

As a college counselor, I highly recommend looking into possible SAT Subject Tests for your child to take. These can help your child's applications by providing objective data points that demonstrate mastery of high school content.