Constant FLUIDS 

pub const FLUIDS: CapabilityKey<dyn ResourceHandler<FluidResource>>;

Fluid tanks, as NeoForge 26.3’s Capabilities.Fluid.BLOCK: a block’s tanks as one side reaches them, numbered from 0 for that side, as ITEMS’ slots. The game answers it for a mod block’s tanks (block_entity_fluids) and for a cauldron: one tank holding the content its block has, vanilla’s water (a bucket in thirds) and lava (a bucket only whole) or a content a mod registered through register_cauldron_fluid_content, filled and drained by whole levels; an empty cauldron takes any fluid that has a content. A cauldron has no block entity, so it is answered by position. Powder snow is no fluid: its cauldron has no content and no answer.