XSPEC allows users to fit data with models constructed from individual components. These components may be either additive, multiplicative, mixing, or convolution. Multiplicative components multiply the model by an energy-dependent factor. Mixing components link models for different datagroups. Convolution models modify the current model. Multiplicative, mixing, and convolution 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, delcomp, and editmod. Model components can be defined interactively using mdefine.
The plasma emission and photoelectric absorption models require an assumption about relative elemental abundances. These relative abundances can be reset using abund. xsect can be used to switch between different compilations of photoelectric absorption cross-sections.