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.