XSPEC allows users to fit data with models constructed from individual components. These components may be either additive, multiplicative, or mixing. Multiplicative components multiply the model by an energy-dependent factor. Mixing components link models for different datagroups. Multiplicative components can act on individual components, on groups of components, or on the entire model. The model command defines the model to be used and prompts for the starting values of its parameters. The user also can set the allowed range of the parameter. One parameter can be fixed to be a multiple of another or to be another plus a constant. Parameters can be unlinked using the untie command.
The value of an individual parameter can be changed with the command newpar. Note that newpar 0 will print out the current parameter values. Parameters can be fixed at their current value with the freeze command and allowed to vary freely with the thaw command. Individual components can be added or subtracted from the model using addcomp and delcomp.