We’ll come back to this in the feedback webinar, but let me tell it already here:
It’s about the _atom_site_occupancy variable. That variable tells which fraction of a position coordinate is occupied by the given element. For DFT input, it must be 1: either there is an atom at that coordinate, or there is no atom at all. In experimental cifs, however, you can have partial or mixed occupancy: an element occupying only, say, 37% of all availabe sites. Or two elements appearing at the same site, in a 29/71% ratio.
For the neptunite cif by Cannillo et al, you see this happening: