It does not define you.
THE ACT vs. THE SAT: WHICH ONE IS BETTER FOR YOUR CHILD?
The two exams are not equal; nor do the two exams suit all students. While exceptions, of course, do exist, I have found that one exam appeals to one kind of learner, and the other exam appeals to another. The first step in this process is to determine the test that is better suited to the student. I do not believe that one test is easier than the other. While one is more forgiving of breadth of content, the other is more brutal with its curve; while one may have more challenging material, the other requires a rapid-fire response to questions. I highly recommend that a student do a diagnostic of each of the tests and then, after reviewing the results, select the one on which to focus exclusively.
Some colleges and universities require subject tests to augment the ACT or the SAT. Students must plan ahead here. If a student, for instance, wishes to take the Physics subject test, he or she should sign up to take that test right after he or she has completed the Physics course. Some students might finish a particular course one year, fail to sign up, and then take the test one year later - long after they may have forgotten the material. So, after each year in high school, students should assess if they should take a subject test that focuses on a course they just finished.