Constant ENTITY_FLUIDS 

pub const ENTITY_FLUIDS: EntityCapabilityKey<dyn ResourceHandler<FluidResource>, Option<Direction>>;

An entity’s fluids, as NeoForge 26.3’s Capabilities.Fluid.ENTITY, asked for the side of the entity a lookup reaches it from, or none for no particular side. Vanilla has no entity that holds a fluid, so only mods answer it.