Every scenario card does not have instructions from depleted off-board artillery. Does it appear in other games(Paths To Hell or A Las Barricadas!)?
The escnarios don't need instructions for off-board artillery, you check the OOB, if have off-board artillery, depending if it's full-stregth or depleted you must check the value in the "Off Board Artillery Chart" looking for the caliber on the table, crossing the data depleted/full-strength and the caliber you will get the off-board value.
For example, if the OOB of the scenario indicates that we have a off-board depleted 100mm artillery, we would go to the "Off Board Artillery Chart" and cross the data between "depleted" and the caliber, in this case "100mm" and it gives us a value of "5".