If HISTORY_FILE is set, then the command history is saved in this file rather than in memory. The advantage to this is that the size of this file is limited by your disk space and doesn't increase the memory usage of IRCII. The disadvantage is that history access might be slower (or it might not). To disable the HISTORY_FILE, set it to <EMPTY>. To completely disable the command history functions, you must also set HISTORY to 0.
See Also: set/history