If the space group tables lists a site with an x, y, or z, ie not zero or a fractional value, then it can take on any value. Beware of sites such as R3 1a, which is x,x,x. In that case, x can be any value, but every fractional ordinate must be the same value.
The decision as to whether the data support the refinement of atomic coordinates is left as an exercise to the reader.