The RZ package uses Fortran I/O, unless the option 'C' is specified on the calls to RZOPEN, RZMAKE and RZFILE. If C I/O is used, the C file pointer is returned by the routine RZOPEN. Thus the LUN parameter must not be a constant.
On most machines, the file pointer returned by the C library is a small positive integer. To avoid possible conflicts with Fortran logical units, the RZ package adds 1000 to such pointers. This is handled internally by the RZ package and has no impact on the user.