If you set the fastf option in $SYSTEM group to .true. (this option is turned on by default) and run PC GAMESS/Firefly under OS & filesystem supporting large files, all files handled by FSF routines may have their size up to the native limits of 64-bit filesystems. Otherwise, large files will be automatically split into several smaller chunks.
FSF is an abbreviation of the Fast System Files - a powerful set of fast non-Fortran file
The DRTFILE sequential access file is always handled by the standard file