From: Carnegie, Australia
From my understanding support units is the way all those elements that do not have a direct combat role are modeled in the game.
For example, in a 41A Soviet Rifle Div approx 55% of the personel are in support unit. This covers Medical, Veterinary, Bridging, Security, Bakery, Signals, HQ, Supply etc.
Not sure this helps...
If I try to click on the scroll bar of a screenshot one more time I will scream.