If he is a fighter pilot, it might be his personal "Call Sign". These informal call signs are often variations on their own name or personal characteristics. Those familiar with him (the fighter community is a small community, so each name is unique and not a daily changing call sign as is the "official" Call Sign off a classified flight Call Sign roster) is how most everyone in the closely knit fighter community knows him.
My guess is, "Cain" is the General's last name, and "Raisin" is his personal call sign name, viz. "Raisin Cain".
The members in his unit would recognize immediately who is on the radio and in combat, but the enemy wouldn't so the unit reaction would be quicker and more tailored to rank and personal experience/authority than a daily rotating generic Call Sign which is easily forgettable.
It is his "personal" handle, and it often travels with the pilot for many years. That is the name he would be "known" as by and especially in combat - by his fellow pilots and other aircrew who know just who he is when in a tight situation requiring rapid and precise communication in a fluid situation. Remember, you can't see his face, rank, or position on the radio and there are a lot of people on the frequency. All you have is a Call Sign for identification and there is a lot of chaos in combat.
His nickname Call Sign would be unique - as would the other member's personal Call Signs in the unit.
I assumed it would be unique. I assumed it was a “handle”. I just thought it was soooo nifty and appropriate for a warrior.
https://www.archives.gov/files/declassification/iscap/pdf/2012-042-doc3.pdf