What does it mean to be coded E for retention?

I was sent to Korea for my first post right after AIT although I had put Vietnam on my dream sheet. I later learned that my MOS was a "critical MOS." Guess that explains why the recruiter push it so hard as it was not what I wanted but it was a long school, that sold me. Nearing the end of my 1st 13th months in Korea I was once again presented with the waste of paper dream sheet, I entered: 1) Vietnam {again}; 2) Korea and 3) Korea. Anything to avoid state-side or Eur assignment. Well, I spent nearly 27 months in Korea, 1968-1970. Go figure. Then, while they were drafting men at higher numbers than since WWII they were giving guys with an enlistment of 3 or 4 years an "early out" so I got out 6 months early. (I am not sure if this applied to all the US guys as well or just RA.) Why keep the well trained soldiers and replace him with raw draftees? Again, go figure. Gotta love the Army! (Which I did by the way,)

Without knowing which specific code he's referring to, I think he's saying that all the requisitions are coded for reenlistment for use by Retention management branch.