XSPEC reads in spectra from spectral files using the data command. Several files may be specified in one command. A particular data set in use may be replaced by another or dropped entirely. The input data file contains pointers to background, redistribution and auxiliary response files, but these pointers may be overridden by the back, resp, and arf commands. All these commands have the same syntax as data. PHA channels may be left out of fitting using the ignore command and included again using the notice command. These commands have a syntax allowing the same channels to be specified for more than one input file. An auxiliary background file, called the correction file, also can be included using the çorfile command. Its use is described in the section on fitting.