Per input file:

Once per command:
        [/BY var_list[({D|A})] [var_list[({D|A})]…]]

ADD FILES adds cases from multiple input files. The output, which replaces the active dataset, consists all of the cases in all of the input files.

ADD FILES shares the bulk of its syntax with other PSPP commands for combining multiple data files. See Common Syntax, above, for an explanation of this common syntax.

When BY is not used, the output of ADD FILES consists of all the cases from the first input file specified, followed by all the cases from the second file specified, and so on. When BY is used, the output is additionally sorted on the BY variables.

When ADD FILES creates an output case, variables that are not part of the input file from which the case was drawn are set to the system-missing value for numeric variables or spaces for string variables.