For what are they actually?
Placing more Batterys in it?

4 slots next to Electro Furnace
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These slots are for upgrades,
Transformer upgrade : increases voltage accepted by machine by one tier. (2 makes it accept HV)
Overclocker upgrade : increases machine speed, at the cost of more energy.
Battery storage upgrade : increases internal storage by 10k.
all upgrades are stackable up to 64for recipes check industrial wiki.