The GRE revised general test and Subject tests vary in their objective and content. The GRE revised general test is intended primarily for finding out the applicants reasoning skills (Verbal and Quantitative) and analytical skills (AWA). It also helps admission panels to compare students who come from various academic backgrounds. The GRE subject test on the other hand is intended at finding out the subject knowledge of an applicant. The subject test is offered in 8 separate disciplines such as Biochemistry, Cell and Molecular Biology, Biology, Chemistry, Literature in English, Mathematics, Physics and Psychology.
Most schools don’t ask for a GRE subject test score, so it is safe to take only the revised general test unless and until the school of your choice asks for subject test score.