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

[Gregorius (Gregorius Techneticies)]

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.