In Open PL/I, a file is a source of input data or a target for data output. Files are referenced in I/O statements by using a file name declared with the FILE attribute. The file reference actually refers to a file constant or to a file variable that has been assigned a value.
Each file used by the program must be declared in exactly one module in the program. That module must either declare the file with the GLOBALDEF attribute or must be compiled with the -defext option.