This flag indicates if the machine reactance should be calculated using all transient parameters (xdss, xds, xd, Tdss, Tds), or if just the subtransient reactance should be used for harmonics. There is only a difference around nominal frequency. For higher frequencies, the exact reactance approximates the subtransient reactance anyway. The typical application are SSR studies, in which the correct representation of impedances around nominal frequency are very
important.