Well, the requirements are spelled out in the B56.1 standards, but in general the overhead guard will protect you from falling objects. I feel like most impacts tend to be partial loads (like a few items from the top of the pallet stack) and the guard should handle that type of impact. A worthy (common sense) disclaimer is that if the objects are small they can fall between the grating.

The overall point I believe they are making is that the guard is not designed for very heavy, very hard, full load capacity hits from elevation. They want you to know the limitations of the guard and therefore take precautions to keep the loads from falling. If your top pallet is stretch wrapped or tightly packaged, so it will fall as a complete 3,000 lb. 4x4x4' cube the lift's guard may not handle that *complete* load's impact, depending on your lifts basic chassis capacity and the height of the fall. You see I have to specify the size of the load, the weight, the capacity of your trucks, etc. just to put this in context... that is why I believe they are not more specific, because there is so much to consider in each scenario and they do not want the liability of putting approximated impact protection... they defer you to the ANSI B56.1 standards which requires a little legwork and your truck data to figure the expected fall protection.

In your case, if you are looking to limit your liability and improve the safe handling of these loads I would suggest extending the backrest to comply with the 1910.178 standards and head off the likelihood of a fall altogether.