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What is the first slot in the gas station GUI for?

If you sneak and right click the gas station, it opens an admin GUI, where you can set an item and a fluid amount (You have to place the gas station yourself to access it).
The slot you asked for is the payment slot. The thing next to the slot shows you how many fuel you get for this item.

The slot only does something if you set a price for your fuel.

Thanks! Also, who can access the admin GUI? I think only the person who placed it should be allowed to.

Exactly. But if you are OP on your server you can also access it.

Great! I wanted to do this on my server, but I was worried about people removing the price from my gas