I mean this slot:

What is it for? What can I put into it?

Hi, it's the auxiliary slot, put a Hopper or Factory Hopper in there to enable that the Furnace automatically draws items from inventories at the input side (marked blue). It's also mentioned in the manual (install Vazkii's Patchouli for that), but I sense the slot should somehow be marked with a shaded symbol for better recognition.

@stfwi Thanks for the reply! I didn't know that Patchouli is supported, it seems to be pretty rare.