Crates is a buyable item that can be usable by a key only. Crates can also be rarely found in-game, they come in different tiers of rarity such as:
Crate Type (Key type)
Crate (Key)
Weapon Crate (Key)
Halloween Bucket (nothing)
Christmas Present (Scissors)
Retro Crate (Key, only when it's retro week)
Goodie Bag of Solarness
Ancient Forsaken Crate (Ancient Forsaken Key)
Golden Holiday Crate of Awesomeness (Key)
Magma Crate of Ever Flames
A Really Big Crate (Key)
Diablo Crate of Unforgiving Terror (Ancient Forsaken Key)
Oridian Crate Of Juvinial/Oolah (Bedamned Key/Oolah key)
Egg Basket (Nothing)
Legendary Crate (Ancient Forsaken Key)
Special Silver Gift of Commemoration(Specialization Administrative Key)
-More Crates/Keys Will Be Added, We Don't Know all Crate
Opening crates of any kind gives you items (whether they're weapons, paints, clothing, or attachments) ranging from ultra-rares to more common items such as attachments.
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HINTS
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You Can Trade Crates For Something Good, Type: Itens,Weapons,Clothing,Guns,Other Crates ( Rarely )