I was an "RA" (regular Army) and in for three years.
"ER" is "enlisted reserve" and was in for SIX MONTHS !
They did Basic and went to an MOS training holding company to play with themselves while we and the "US" (draftees) were shuffled off to MOS schools (military occupational specialty), which, in 1965 was generally some kind of combat specialty (I was sent to radio teletype school).
After getting trained, we were shipped out to our duty stations which was more than Likely Viet Nam.
I don't know why, but I was sent to Korea, which is the worst and greatest place for a 17 year old virgin.
We never thought much of reservists back in those days.
During my time (68-76) an enlistment (or draft) called for a total6 year commitment. For draftees it was 2 yrs active and 4 years inactive reserve. For enlistees in the AF or Navy, for example, it was 4 years active and 2 years inactive reserve.