It is used after option 10 has calculated the wavelength scales.
The wavelength-consistency check is confined to those areas beyond the range within which lines have been identified. It therefore only corrects the very ends of the orders wavelength scales.
These are the regions where problems are most likely to occur, as the polynomial fits can become unstable when a high number of coefficients has been used, and there are no fitted points for a substantial fraction of the order (e.g. first 20%).
Martin Clayton