Classified Items are rare but powerful support items which help advance the player's progression.
They can be obtained from Submarine Dives (within the mysterious treasure slot at a low chance), be a reward from the Boom Pass, or can occasionally be purchased from the Trader.
You can store up to 2 Classified Items of each type at a time. If you already have 2 of a Classified Item, discovering a third of the same type from a finished Submarine dive will make it impossible to collect the treasure until one of the currently stored ones is used.
If you cannot use an item for whatever reason, you will be able to destroy them so you can claim the rewards and free up the Submarine. This doesn't apply to any type of safe.
Items bought through the store or obtained from the Boom Pass can exceed this limit.
There are currently 6 different Classified Items.
List of all items
Attack Plan
The Attack Plan instantly completes any upgrade associated with the Armory, save for mines.
The Building Blueprint instantly completes any building upgrade.
Safes
Resource Safe fills up the player's resource storages of their respective types.
A Gold Safe
A Wood Safe
A Stone Safe
An Iron Safe
Strategies
General
Always send the Submarine on any dive location that contains mysterious items. It could potentially uncover one and give you a huge advantage in one way or the other.
Attack Plans and Building Blueprints
To get the most out of these items, they should be used as soon as the upgrade is underway.
For example, if a building upgrade would take several days, it can be used as soon as the upgrade begins to skip it. This practice is recommended on buildings critical to your defense/offense such as the Shock Launcher and Landing Craft since they can't function normally when upgrading. Secondary priorities include the Armory, Sculptor, and Weapon Lab. It is not recommended to use them on any other buildings such as Resource Storages/Producers, Gunboat and Radar as it effectively defeats its purpose.
Safes
To get the most value, use them when their respective storage has been almost or completely depleted.
They should only be used if needed due to a resource shortage on an available upgrade, which should be done right after to prevent them from being stolen.
Trivia
Classified items were inspired from the Magic Items in Clash of Clans, a similar Supercell title.
Safes are mentioned to be directly relevant to the Trader as she can somehow uncover hidden repositories in the archipelago for the player, implying that the item itself is merely a set of coordinates.
The Building Blueprint seems to be associated with the Blackguard. As the Trader also seems to be associated with classified items in general, it may support the theory that she herself is Colonel Gearheart.
It also implies that the Blackguard possesses extremely advanced construction technology allowing them to fabricate buildings with terrifying ease instead of beginning from scratch.
The Attack Plan's name is inaccurate, as in real life, a stolen attack plan would only give an advantage against hostile forces for a moment and not permanently, which it actually doesn't do in-game. A more accurate name would be "Technology Blueprint".