The original goal of taking rates in the first place, was to make the Navy standardized with its sister branches by having an MOS-like system by way of Navy Occupational Specialty (NOS) codes that replace our ratings. However, it was overlooked that the Navy does already have an MOS system, not to be confused with the NOS nonsense. We have what is called an Navy Enlisted Classification (NEC) that serves the the exact same purpose as MOS.
For example, you have 0300 Basic Infantry Marine, 0311 Rifleman, 0317 Scout Sniper 0321 Reconnaissance Marine, 0323 Reconnaissance Marine, Parachute Qualified, 0324, Reconnaissance Marine, Combatant Diver Qualified, 0326 Reconnaissance Marine, Parachute and Combatant Diver Qualified, 0331 Machine Gunner, 0341 Mortarman, 0351 Infantry Assault Marine, 0365 Infantry Squad Leader, 0369 Infantry Unit Leader - just to name a few, and all of these fall under "Infantryman" in the Marine Corps. Hypothetically speaking, your Marine "rating" would be Infantryman where as your Designator would 03xx << Look familiar?.
Looking back at the Navy: HM-8401 Search and Rescue Medical Technician, HM-8402 Submarine Force Independent Duty Corpsman, HM-8403 Fleet Marine Force Reconnaissance Independent Duty Corpsman, HM-8404 Field Medical Service Technician, HM-8406 Aerospace Medical Technician, HM-8407 Radiation Health Technician, HM-8408 Cardiovascular Technician, HM-8409 Aerospace Physiology Technician, HM 8410 Bio-Medical Equipment Technician, HM-8416 Nuclear Medicine Technologist, HM-8425 Surface Force Independent Duty Corpsman, HM-8427 Fleet Marine Force Reconnaissance Corpsman, HM-8432 Preventive Medicine Technician, HM-8434 Hemodialysis/Apheresis Technician, HM-8451 Basic X Ray Technician, HM-8452 Advanced X-Ray Technician, HM-8454 Electroneurodiagnostic Technologist, HM-8463 Optician, HM-8466 Physical Therapy Technician, HM-8467 Occupational Therapy Assistant, HM-8472 Biomedical Photography Technician, HM-8482 Pharmacy Technician , HM-8483 Surgical Technologist, HM-8485 Psychiatry Technician, HM-8486 Urology Technician,cHM-8489 Orthopedic Cast Room Technician, HM-8493 Medical Deep Sea Diving Technician, HM-8494 Deep Sea Diving Independent Duty Corpsman, HM-8496 Mortician, HM-8503 Histopathology Technician, HM-8505 Cytotechnologist, HM-8506 Medical Laboratory Technician, Advanced, HM-8541 Respiratory Therapy Technician, HM-8701 Dental Assistant, HM 8707 Field Service Dental Technician, HM-8708 Dental Hygienist, HM 8752 Dental Laboratory Technician, Basic, HM 8753 Dental Laboratory Technician, Advanced and HM 8765 Dental Laboratory Technician, Maxillofacial is every NEC that falls under the Hospital Corpsman rating. But rather than remember all of that, we can just call him/her HM2, thus recognizing what they do without going into details as well as knowing their rank. For more information on Navy NECs equivalent to Air Force/Army/Marine Corps MOS' reference the Manual of Navy Enlisted Manpower and Personnel Classifications and Occupational Standards Volume II Navy Enlisted Classification (NECs) NAVPERS 18068F.