Um, shells on a ballistic arc from over the horizon are irrelevant to naval battles. Simply put, when both platforms are constantly moving, it's practically impossible for ships to hit each other at range. The only way to achieve hits are to seperately disable the other ship's propulsion and steering, or get extremely close (i.e. inside LoS). You simply can't hit a ship from over the horizon with a ballistic projectile.
There was a day-long gun duel at range in WW2 where neither side managed a hit. There's a reason all the gun ships went away, and we depend on planes and missiles now. Guns simply don't work anymore.
It doesn't appear the government has asserted the document is classified - that's a term of art meaning that the release of the data would compromise national security in some way. Instead, they've declared that the data was marked For Official Use Only, which means the data is unclassified, but not for wide dissemination.
Let's break it down. We have classified information, which is data that, if released, would affect the national security. This determination is made by the President or directly appointed representatives only - I think Deputy Secretary of the Army/Navy/whatever is the lowest level with classification authority. Everyone else is merely applying the policies as determined by the originating authority. So I, as a low-level contractor, cannot unilaterally decide to classify a piece of information. Instead, I apply a predetermined set of rules (does it come from system A or mention topic B) to data I sort, and mark it appropriately.
For classified data, there are three broad groupings - Confidential, Secret, and Top Secret. Secret has a subcategory of NoForn - not to be shared with foreign governments. Top Secret has a bazillion "code-word" subcategories - my favorite was Cosmic Top Secret.
There is also a category of unclassifed information that should not be in wide release. This is information that would not impact national security, but should still be controlled. The classic example is Personally Identifiable Information (PII). Leaking your address and social wouldn't impact national security, and neither would leaking your medical records or job evaluations. But I think we'd all agree this information should be kept out of the public eye, so it's marked FOUO. Not classified, but still not for dissemination.
The other category of FOUO information tends to be operational details for a command. This would include unit movements, detailed meeting schedules, specific evaluation criteria, etc. The stuff that, in a corporation, would be tagged Company Propriatary.
Finally, there is unclassified information that is treated as classified. This is generally any build media used for classified systems. The media itself isn't classified - it's straight from the vender. But once we have it, we treat it as if it's classified at the level of the system it was used to build. That way, no one can modify the unclassified source material without already having access to the classifed data.
New York... when civilization falls apart, remember, we were way ahead of you. - David Letterman