The measure of absolute structure (enantiomorph or polarity) 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 (estimated standard deviation) u must
be supplied. The item 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.