Sorry for incorrect information.
Fortran I/O Units Usage by CHARMM
In order to keep CHARMM as machine independent as possible, all
specification of files is done through Fortran unit numbers. Two unit
numbers have special signifigance, 5 and 6. Unit 5 is the command file
interpreted by CHARMM. Unit 6 is the output file for all printed
messages. As commands are read from unit 5, they are echoed on unit 6.
All other unit numbers have no predefined meaning. The CHARMM OPEN
command may be used to assign files to units.
It just follows fortran's I/O rules.