According to the AMA Manual of Style website, the first edition of the style guide was intended to be used as an in-house editorial manual for the scientific journals published by the American Medical Association. Many journals are still using the AMA style, although the Uniform Requirements of the ICMJE (aka "Vancouver Style") is an increasingly popular alternative.
Most instructors will tell you which citation style to use for their assignments. If your instructor doesn't have a preference, you should use the citation style most appropriate for your career or field.
The citation style you use should be based on the journal into which you are seeking publication. Guidelines for publication can be found on the journal's website.