Mostly for your purposes the distinction is from the commissioning source.

By and large with a few exceptions all those ROTC, V-12 and earlier V-7 College Program, and V-5 Aviation Cadet programs were producing Ensigns with Reserve commissions. It was understood that the Navy wouldnt need this many officers forever and the peacetime establishment could at least in large part be met by Annapolis which was producing most of the regular officers, though at a greatly enlarged and accelerated pace. Reserve is what the R in ROTC stands for in the end! Even if the programs and their outputs have changed a lot since in the experience of graduates, the new USNR Ensign from an ROTC unit would train briefly and then go to just a few weeks drill each summer on an installation or ship. While some were called to active duty as war threatened, the majority of the peacetime officer corps were regular officers from Annapolis.

There was some bias though in assignments from time to time. Admiral King even had to explicitly draft a memo requiring Flag Officers to give preference to USNR officers for their staffs, thus releasing more USN officers for duty at sea. And as the war wound down it would be obvious most USNR officers would be taken off active duty and sent on their way just like reservists through time, and those that wanted to stay on active duty as Reservists or drop the R and go Regular Navy had to make their case very strongly. Many did but the Navy and DoD as a whole was greatly reduced of course.

By the end as well the vast majority (between 70 and 80%) of the US Naval establishment by 1945 was technically Navy Reserve. And the vast majority of those officers doing good service, and many in the not too glamorous jobs like commanding support craft, patrol craft, and filling out staffs.