I would usually define "slot" as a fixed opening, for example a slot that is sized for people to enter coins into a machine. A shelf wouldn't qualify unless it had defined areas for each individual ball, so it wasn't just one long continuous bench. I would also use "space" and maybe also "spot"

Hello Lily! I agree with the other answers but I would like to add that a slot is definitely a space that is sized specifically for something, for example cubicles made to fit things uniformly on a shelf.. The word "slot" is also used to refer to specific times in a schedule. I would call open space on a shelf: "open shelf space" or "shelf-space" or "available space on that shelf over there" If you are speaking about a space that is left or created for something specific I would say: "the space allocated for...." or "the space allotted for..." or "the space purposed for..." I hope that helps!

Thank you so much for the elaborate answer and examples, Duncan :) Thank you so much, Alyosha :)

I agree with Alyosha and Jennifer, I would normally use space. I would use slot if there was a specific way to locate something on the shelf, to slot it in. That item would have a slot. It can also be used as a synonym for cram, or find a place where there isn't much room available. - Is there space for another book on the shelf? - How much shelf space do we have left? - Can you just slot it in on the end? - Put it back in its slot. - Do we need to create a slot for it? - Do we need to make space for it? - Do we need to make some room for it? - Can you put it back in its spot, please?

space!