The Reserve Array node reserves memory for an array, but it doesn’t construct the array for you.

So, for example, if you try to use Set in Array you may find that it doesn’t work, because the array element you’re trying to set doesn’t exist yet. In this tree, Reserve Array gives me an array with one element. So trying to set the second element has no effect.

I’d have to use Push on Array here to get the desired result:

Or I could make sure the array elements exist like this:

I could also Resize the array after I reserve it.