Hi Stas and wikidatans,
the coordinates with (sub)millimeter precision might be imported from
external dataset. With precise geodesic measurements, it is technically
possible to reach such precision, yet it often decrease significantly by
transformation to WGS84. Some places like important peaks or astronomy
observatories can be located with such precision.
So I am generally against limiting the number of digits too much, but limit
it to something like millimeter precision sounds reasonable.
Regards,
Jan