To become a fellow of the American College of Surgeons (FACS) or the American College of Phsycians (FACP) you have to be board- certified by the respective college. That grants you Diplomate or Member status. I think that is what MACS means. Fellowship is granted uopn proving evidence of scholarly work involving research in your respective field and Academic teaching involvement. I have heard that it is much easier to get FACS for surgeons than FACP for Internists, respectively. Every specialty has its own boards and requirements for fellowship (ie: ACEP- Emergency Physicians, ACFP, Family PRactice, FACC, Cardiology, etc. The Osteopathic profession also has its own boards (ie: FACOI-Fellow, American College of Osteopathic Internists, FACOS-Osteopathic Surgeons.)D.O.s are eligible for fellowship into osteopathic and allopathic Colleges after completing set requirements ( for one or the other. They are usually interchangeable). ie: FACS or FACOS grants you eligiblity for the other and vice versa.
Hope this helps.