There has always been quite a muddy line about the difference between "enlisted" personnel and "officers" in Starfleet.
IIRC, in TOS, everyone had officer ranks. Sure, they had "Transporter Chief" or "Security Chief" or "Yeoman", but those seemed to be more titles than ranks or grades. Everyone seemed to be out of Starfleet Academy.
The movies were a little more enlisted and non-com friendly. They had the red/white jumpsuits and there was a clear line between them and the officers, who wore the full red/black tunics. No real understanding of the difference in the process leading them to this career path, however.
Then we kind of went back to officers only in TNG and beyond, with the one apparent exception of O'Brien, who was a Chief Petty Officer (although he had Lieutenant rank devices in TNG when he was on the bridge or operated the transporter), who later got his own formal Chief's rank device in DS9 - and the ONLY one that I can recall, and completely different from anything any other officer has ever worn. But I think he also went through the Academy.
Other odd-man-out rank devices included Kosinski in "Where No One Has Gone Before", who wore the uniform, but seemed to operate in the role as a civilian consultant who lorded it over the other senior officers in engineering, and Director Sloan (Section 31) who had Starfleet Captains' pips with a solid bar underneath and jurisdiction over Sisko on his own station, yet was not referred to as "Captain", "Fleet Captain" or "Commodore", as the device might have implied.
So what does "enlistment" vs. "commission" mean in the Starfleet world? Don't know with absolute certainty. It's never been adequately explained as far as I know. Maybe just "enlisting" into Starfleet without going to the Academy gets you put in as a baseline crewman; 2 years in the Academy gets you to a non-commissioned officer, starting at Petty Officer; an extra year on top of that for specialist training to get a Warrant Officer grade (Kosinski, perhaps?) and a full 4 years gets you a commission as Ensign, or possibly Lt.JG (Junior Grade), depending on your performance.