I agree that some rules, as written, as difficult to discern. Sometimes it takes a knowledge of other rules for complete understanding of a particular rule. I think the writers of the rulebook did an outstanding job of trying to keep the verbiage down to a minimum and that is important in a 500 page rulebook. So you have to extrapolate the answer. Any unit that cowers is marked with a prep fire or final fire counter therefore an unmarked unit has not cowered. When a fire group of multiple units cowers use random selection to determine which units are marked as prep fired or final fired and therefore they have cowered and the unmarked units have not.
To respond to Houtje's comment about everyone cowered in the FG since the attack cowered I would suggest thinking about it a bit differently. Cower is what a unit does, not what an attack does. The column shift is the penalty that is paid for a unit cowering since the attack would not be as effective if everyone is not engaging the enemy. It is a relatively simple protocol to follow. There is no way in that simple process to "partially" cower when there are multiple units involved in the attack. If you wanted to, I guess you could do RS before resolving the attack and based on the number of cowering units adjust for more or less column shifts but that becomes unwieldy compared to the simple, elegant, penalty for a unit, or units, not engaging the enemy as expected. Is the same penalty for one unit or all the units cowering unrealistic? Perhaps, but for playability purposes and simulated effects it work perfectly. There is no need to know how many soldiers cowered just that some of them did and it was enough to effect the attack. Works for me.
At the level that ASL portrays, these type of random events are part of what makes the game so engaging. There is always something that could go wrong. In most war games I can add up the numbers, look at the chart, and calculate the probability of success. Even the most powerful attack you can muster can come back and bite you with a sniper taking out an important asset.
Mike