Constant ITEM_FLUIDS 

pub const ITEM_FLUIDS: ItemCapabilityKey<dyn ResourceHandler<FluidResource>>;

Fluid held by an item, as NeoForge 26.3’s Capabilities.Fluid.ITEM: a handler for the item at one place, whose changes rewrite the item there with an exchange of one, so from a stack of more than one the changed item splits off. The game answers it for vanilla’s buckets (NeoForge’s BucketResourceHandler): one tank of a bucket, filled and drained whole, an empty bucket taking plain water or lava.