Pymatgen `PeriodicNeighbor` class. What does `index` attribute exactly mean?

I’ve been recently looking at PeriodicNeighbor class of pymatgen noticing it comes with some attributes (site, distance, index, image). The index in particular is described as ’ Index within structure… Defaults to 0.’ , but I don’t understand what does it exactly refer to. I have noticed this attribute is taken into conisderation in CGCNN model indexing neighbors of each atom in the unit cell, but again, I’m not sure what this indexing exactly refers to in such context.