CLASS uses two files of data; one for input and one for output, which may be the same actual file. The input file is only used to read. An observation contains several independent subsections. These file are defined by the command FILE (IN,OUT,BOTH) Filename, possibly followed by NEW if a new file is to be initialized. The default extension of files is .30m. This can be changed with SET EXTENSION .my_extension.
CLASS also keeps two log files, named class.log and class.mes. They are created at CLASS start in the default directory $HOME/.gag/logs. This directory is defined in the SIC logical gag_log: which can be customized in the user customization file: $HOME/.gag.dico. The log files may be used to keep track of a batch or interactive work.