This option tells FPDoc what input file should be used. The argument can be just a filename, but can also be a complete compiler command-line with options that concern the scanning of the Pascal source: defines, include files, syntax options, as they would be specified to the Free Pascal compiler when compiling the file. If a complete command is used, then it should be enclosed in single or double quotes, so the shell will not break them in parts.
It is possible to specify multiple input commands; they will be treated one by one, and documentation will be generated for each of the processed files.