What type - Reserve or Regular - Commission was predicated on whether you were "on scholarship," IOW the US Government was paying your tuition & fees, or not. Regular ROTC students got a monthly stipend - IIRC $50 the first two years & raised to $100 after that. "Contract Students," IOW the oxymoron "Reserve" commission midshipmen, didn't get the stipend until they signed to go "Advanced ROTC" as juniors. "Regulars" had applied for their scholarships, competed & been selected while in HS. "Contracts" were essentially "walk-ons."
If Uncle Sam was paying your way through GT, Auburn, Ole Miss, Vandy, Cornell, UNC, USCe, Villanova, USC(real), ND, or wherever, you were going to get a Regular Commission & owe 4 years Active + 2 years Reserve as payback. For us Navy types, AV & NUC/SUB & possibly SUPP schools would have tacked on additional payback years.
If Uncle Sam was not paying you to attend classes, you got a Reserve Commission and owed 3 years Active and 3 years Reserve.
Before the end of your initial obligated active service, Navy Reservists on Active Duty (not to be confused with the Inactive Drilling Reservists on two weeks annual training "Active Duty") were eligible to apply to "Augment" into the Regular Force. That was my initial roadmap from USNR==>USN before I punched out after six years & went USNR as a Drilling Reservist for the next 24 years, including two "Presidential Selected Reserve Callups," or PSRCs, for ODS/ODS One & again for Bosnia.
FYI, Regulars start receiving their Retirement Pay immediately upon retirement. Reservists don't start receiving Retirement Pay until they turn 60 years old. Interestingly, once a Reservist turns 60 & starts receiving that retirement pay, they get redesignated from USNR (Retired) to USN (Retired), or USAR (Retired) to USA (Retired), et al. "Grey Area Retirees" is a subject for another day.
CAPT DeepSnap, USN (Retired)