fin is a character expression or * naming the file to open. An OPEN statement need not specify a file name. If the file name is not specified, a default name is created.
If you open a unit that is already open without specifying a file name (or with the previous file name), FORTRAN thinks you are reopening the file to change parameters. The file position is not changed. The only parameters you are allowed to change are BLANK (NULL or ZERO) and FORM (FORMATTED or PRINT). To change any other parameters, you must close, then reopen the file.
If you open a unit that is already open, but you specify a different file name, it is as if you closed with the old file name before the open.
If you open a file that is already open, but you specify a different unit, that is an error. This error is not detected by the ERR= option, however, and the program does not terminate abnormally.
If a file is opened with STATUS='SCRATCH', a temporary file is created and opened. See STATUS=sta.