[phenixbb] distance threshold for special positions?

I'd be grateful for pointers to documentation/source regarding phenix.refine's criterion for flagging an atom's coordinates as coinciding with a special position. For shelxl I noticed a distance cutoff of 0.02 to the symmetry equivalent position seems adequate for the data I'm working with. For phenix.refine the threshold is higher but I haven't found documentation for the cutoff used. thanks, Alastair Fyfe