how does the custom items option work

How does the custom items option work because i have no idea what all of the numbers and stuff mean in the custom items menu and the game doesnt tell me either. can you please tell me how it works because i have no idea:

On the lefts are which places a character is in, the top is the items, the slots mean how common an item is for a specific places (the higher the number, the more common it is), slots with nothing mean that the place on the lefts cannot get the item on top, you can change the numbers by clicking them and putting a new one (or nothing, 0)

But that doesn't mean that you can only get a fake item box or banana in first, or a green shell in first or second, or anything like that

And also where did you get that information from

You can only get those items in first is because the other items are blank which means you can't get them.

It does mean that? Idk what you're on about

And he got the information from knowing how the feature works