This was well studied during WWII -- we can thank the US Army for wanting to study everything.
Generally, what we could reasonably interpret as "Coolness Under Fire" improved with combat experience, to a point, and then deteriorated with further combat experience -- this in the context of e.g. the Normandy campaign. The Twilight War probably resembles that campaign much more closely than any recent US Army experience, in that there are not enough troops to rotate units out for extended rests and there is no such thing as being a short timer. I only recall this vaguely, it was in John Ellis's book
On the Front Lines (a.k.a.
At the Sharp End) and I may have sold my copy.
Basically your green troops don't recognized combat hazards and may overreact to things, but with experience they learn not only to distinguish incoming and outcoming fire but also to distinguish things like an incoming shell that is passing over from one that will hit close by. But with further combat exposure they become jumpy and begin overreacting to any potential threat.
So, applying this knowledge, soldiers in Twilight:2000 should have CUF F while civilians should have CUF D.
That might not work so well in the game! But clearly it should not advance with rank regardless of trade & should be coupled to actual combat experience, with variability between individuals because people possess sang froid and resilience in varying degrees.