Loaders and Unloaders are used to load/unload fluids and items from the rocket.
They can be linked directly to a rocket, a rocket assembler, or Docking Pad using a Linking tool.
When linked to a Docking Pad or rocket assembler, any rocket that lands on the Docking Pad or on the landing pad will automatically be linked to the loaders and they will automatically unload/load their payload.