Constant ITEMS 

pub const ITEMS: CapabilityKey<dyn ResourceHandler<ItemResource>>;

Item slots, as NeoForge 26.3’s Capabilities.Item.BLOCK: a block entity’s slots as one side reaches them. The game answers it for every block entity with slots: chests (a double chest’s two halves as one), furnaces, hoppers and the other vanilla containers, by their WorldlyContainer sides, and a mod block entity’s block_entity_container.