The fit and goodness-of-fit test statistics are set using the statistic command. Other fit-minimization algorithms are available, and can be selected using the method command. The various fit methods require first and in some cases second derivatives of the statistic with respect to the parameters. By default XSPEC calculates these analytically, using an approximation for the second derivatives. This may be changed by setting the USE_NUMERICAL_DIFFERENTIATION flag in the user's startup Xspec.init file. The weighting algorithm used to calculate can be altered by the weight command.

Other setting commands modify:

- cosmological parameters used to calculate luminosity (cosmo)
- solar abundances for 18 elements (abund)
- photoionization cross-sections (xsect)