What?
When you middle click on a cargo wagon slot, you can add a filter, so only a certain item can be in that slot. I suggest that there should be a tip in Tips and Tricks about this!Screenshot from the game:
Why?
Factorio doesn't mention its slot-filtering feature anywhere (except for in Settings>Controls, where it is called "Toggle filter"). It is worth mentioning because players can use it to build interesting factories.If this actually is mentioned somewhere in the game... let me know!