The ICMJE style is formally known as the Uniform Requirement for Manuscripts Submitted to Biomedical Journals (URM) and was written by the International Committee of Medical Journal Editors. It is used by many biomedical journals including, but not limited to, the Annals of Internal Medicine, British Medical Journal (BMJ), JAMA (Journal of the American Medical Association), New England Journal of Medicine (NEJM), The Lancet, and PLOS Medicine.
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.