A description of the extinction correction method applied with the data item _refine.ls_extinction_coef. This description should include information about the correction method, either 'Becker-Coppens' or 'Zachariasen' The latter is sometimes referred to as the 'Larson' method even though it employs Zachariasen's formula. The Becker-Coppens procedure is referred to as 'type 1' when correcting secondary extinction dominated by the mosaic spread; as 'type 2' when secondary extinction is dominated by particle size and includes a primary extinction component; and as 'mixed' when there is a mixture of types 1 and 2. For the Becker-Coppens method it is also necessary to set the mosaic distribution as either 'Gaussian' or 'Lorentzian'; and the nature of the extinction as 'isotropic' or 'anisotropic'. Note that if either the 'mixed' or 'anisotropic' corrections are applied the multiple coefficients cannot be contained in *_extinction_coef and must be listed in _refine.details. Ref: Becker, P. J. & Coppens, P. (1974). Acta Cryst. A30, 129-153. Zachariasen, W. H. (1967). Acta Cryst. 23, 558- 564. Larson, A. C. (1967). Acta Cryst. 23, 664-665.