The GRE Subject Tests are designed to help graduate school admission committees and fellowship sponsors assess the qualifications of applicants in specific fields of study. The tests also provide students with an assessment of their own qualifications.
The subjects covered by these tests are:
Scores on the tests are intended to indicate your knowledge of the subject matter emphasized in many undergraduate programs as preparation for graduate study. Since past achievement is usually a good indicator of future performance, the scores are helpful in predicting your success in graduate study.
Because the tests are standardized, the test scores permit comparison of students from different institutions with different undergraduate programs. For some Subject Tests, subscores are provided in addition to the total score; these subscores indicate the strengths and weaknesses of your preparation, and they may help you plan your future studies.
You an take these tests through your ESO/TCO, but the Coast Guard (through DANTES) will only pay for it once unless you're taking it to satisfy a legitimate service requirement (e.g., so you can apply for a Coast Guard-sponsored graduate school).