Greg is on the right track - just a couple of fine points: the tonnage that is referenced is not displacement of the vessel, rather a measure of the volume capacity of the boat - Greg's description is OK, just not the word displacement. A barge made of balsa wood and one made of steel could have the exact same dimensions and therefore the same tonnage. The steel barge would obviously weigh more, sit lower in the water and therefore have a greater displacement. But the tonnage measurement would be the same.
The quote you posted above is accurate. I find nothing contradictory about it. But, "at the option of the owner" could be clarified since it may be the owner's lender that requires it as in your case. "Some states require documented vessels to also display a state registration decal" - That's the decal, not the numbers. Nowhere in that statement does it say the state numbers are to be displayed, only the decal.
The quote you posted above is accurate. I find nothing contradictory about it. But, "at the option of the owner" could be clarified since it may be the owner's lender that requires it as in your case. "Some states require documented vessels to also display a state registration decal" - That's the decal, not the numbers. Nowhere in that statement does it say the state numbers are to be displayed, only the decal.