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Item capabilities are handled differently to Entity/TileEntity capabilities because Items are singletons, i.e. there's only one instance per item type. Instead of Item implementing ICapabilityProvider directly, it has a method (Item#initCapabilities) that creates and returns a separate ICapabilityProvider instance. ItemStack (which implements ICapabilityProvider) calls this method from its constructors and stores the returned ICapabilityProvider, using it in its own implementation of the interface. You can see some examples of items with capabilities here, here, here, here and here. These all use one of two ICapabilityProvider implementations: CapabilityProviderSimple (only implements ICapabilityProvider, doesn't save/load capability data) and CapabilityProviderSerializable (implements both ICapabilityProvider and INBTSerialializable so it saves/loads capability data).