rtree / hilbert / func New(bufferSize, ary uint64) => what is parameter ary?

I tried New(100,2) for a tree managing 2D rectangles. But that failed... I used New(100,3) which works but I don't know why.

So, what does this parameter specify?

And if I want to manage, let's say 5D "rectangles" I just extend my rectangle definition to 5 dimensions?