Core dictionary (coreCIF) version 2.4.5
The measure of absolute structure as defined by Flack (1983). For centrosymmetric structures, the only permitted value, if the data name is present, is 'inapplicable', represented by '.' . For noncentrosymmetric structures, the value must lie in the 99.97% Gaussian confidence interval -3u =< x =< 1 + 3u and a standard uncertainty (e.s.d.) u must be supplied. The _enumeration_range of 0.0:1.0 is correctly interpreted as meaning (0.0 - 3u) =< x =< (1.0 + 3u). Ref: Flack, H. D. (1983). Acta Cryst. A39, 876-881.
The permitted range is 0.0 -> 1.0