What is the unused slots of some machine's GUI?

Some slots in GUI of some machines are not used in any recipe, such as fluid input slot of smelter, item output slot of matter fabricator, second and third item input slot, third fluid input slot and second, third item and fluid output slot of matter replicator . Why do these useless slots appear in GUI? And so far?

  1. The Smelters Input Slot for Fluids IS used for the Plant Oils to Frying Oil Recipe.
  2. Item Input for the Matter Fabricator is used for IC2 Scrap and Stuff when doing IC2 UUM in it.
  3. The Matter Stuff has multiple Slots even if unused, just in case I want to make some Recipe that makes use of those Slots.
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Oh, maybe it’s because I don’t eat potato chips and fries so much that I forget this recipe… But the material replicator does have some slot and it has no use.

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