I still don’t understand these, and the Wikipedia article doesn’t help. Ratings are kind of a mix of your rank and your Navy mos, but not for everybody? How are you supposed to know how you refer to anyone? Navy ranks are already ass-backwards as it is. We don’t call each other ‘Cavalry scout’ or ‘supply sergeant’ or ‘NBC dude’... You just say’ sergeant’ or ‘specialist’ or ‘sir’. Anyone can recognize your rank, no matter what unit you’re from or what you do.
If you are NOT NAVY then just KEEP YOUR NOSE OUT OF THIS!
I can explain it as I was an Army NG after I got out and Army ranks confused me because in the Navy as E-4 I was a NCO all E-4 - E-9 in the Navy are Petty Officers. Rank E-7 is Chief Petty Officer, E-8 is Senior Chief, and E-9 being Master Chief Petty officer.
When identifying yourself to others or in paper work you use you rating {MOS abbreviation being two or three letters followed by rank. For example MM3 Jones is Machinist Mate Third Class Jones. Third Class meaning Third Class Petty Officer the most junior NCO. A first class Petty Officer is E-6.
In the E-3 and below ranks can be in one of three Apprentice Groups falling under Seaman which is deck crew, electronics, cooks, yeoman, personalman, etc. Airman covers all involved directly with aircraft operations. Fireman covers the Engineering Department meaning propulsion and it's auxiliaries. I was an Army NG at rank of E-4 Spec 4 with no MOS. I was in a Howitzer Battery and drove the ammo truck. It was a one year enlistment.
One last thing. In our dress blues or whites we can tell our Rating {MOS} by looking at the persons left shoulder. The rank will have a symbol as to their rating. Mine was a propeller meaning Machinist Mate with an Eagle above it and red V below it.
On ship sailors are berthed by division. We pretty much knew who was what. Petty officers wear a crow on their dress uniforms that shows their rate and specialty. As part of the “old” Navy that wore dungarees and chambray shirts I’m not familiar with the camo stuff which never made sense on a grey ship. Just as soldiers are called soldiers, sailors are called sailor or petty officer depending on rate.