Since XSPEC can fit simultaneously several sets of data (e.g., from multiple detectors), the data command will allow multiple files to be input. Elsewhere in XSPEC, each set of data is referred to by its associated data set number. <data set number>, distinguishable from the list of file names because it is an integer, is the data set number for the first <filename> in the specification, <data set number>+1 is the data set number for the next file, and so on. If no <data set number> is given in the first <filespec> it is assumed to be one (1), so that the first data file input is data set 1, the next file is data set 2, and so on. A skipped-over argument indicates that the data set for that position (as input in an earlier invocation of d ata) will continue to be used. If the filename input is none, that data set is completely removed and any higher-number data sets are renumbered.
The data command determines the current total number of data sets. If the command line is NOT terminated by a slash (/), the total number of datsets is the largest data set number given for any files explicitly input, or the largest value of a <data set num> argument. If the line is terminated by a slash (/), then the current number of data sets is the previous total number of datsets or the number as determined from the command line, whichever is greater. The exception to this rule is that if there are NO arguments to the data command, then the number of data sets is unchanged. (To remove all the data files from consideration would require a command like data 0.)