I thought this was covered in the FWWC notes section, but a quick scan of the notes did not reveal any reference to how the fire mechanics work in detail, I am guessing the notes discuss the FWWC series in isolation and don't refer to how this series differs from Panzer Campaigns? Of course, if you had never played PzC then you would not be aware the fire results in regard to unit stacking/density could be so different.
There are two factors that Panzer Campaigns players would note that gives us some clues, first in the optional rules there is no Alternate Direct fire rule option, this is because the FWWC titles use this rules fire mechanics by default and second on page 33 of the rules in the "Fire against Soft Targets" section it states.....
"When a stack of soft targets is fired upon, then one randomly selected target in that stack receives the majority of the fire while the other units in that stack receive fire proportional to their strength relative to the total stack."
I know for a fact that this is not exactly the same mechanics as the PzC Alt Direct Fire rule as stated in the PzC manual, but the effects are so similar I doubt anyone playing the two series would notice the difference.
Having these fire effects on by default with no option to use the PzC fire rules was a work of genius by the designer, as it forces any player wishing to be successful playing the various FWWC titles to use the correct tactics of the era or suffer dire consequences.