Deck Loading

I am trying to figure how the deck loading is calculated on GOM work boats. If the deck is rated at 600 lbf/ft^2, what exactly does this mean?
Is this the static load of the cargo on the deck? Is it the maximum loading for the deck, meaning that dynamic forces need to be computed? What about lashing loads?

I was under the impression that the load rating was for static loads and the dynamic and lashing factors were included in the calculations.