[Imp]

Just to clarify & this is only from playing but units are more likely to fire at closer (as in easier to hit targets including moving infantry) than harder to hit targets.
My guess is there is a routine based on chance to hit & inflict damage plus a threat level routine, but I have no idea its just observation.
Examples
I have 2 infantry squads one moving one stationary. The stationary one is far more likely to fire at medium to long range than the moving one as its fire is more accurate.

The squad that doesn't use its AT weapon because it moved or is suppressed & hence will most likely miss.
Also a nice touch on occasion they seem just not to bother then open up in their turn causing a bit of a surprise.

Tank ignores the halftrack at moderate range as its no real threat yet opens up at the tank. Try running enough halftracks past it though & it might decide to take the shot rather than wait for a better target.