Other usage in the Disk Space Usage section of cPanel is calculated by files that are located outside of the home directory that are owned by that user. So it could be files in /tmp or Mailman archives, or backups, etc etc.
If you are trying to track down where your disk space is used up, and you see a lot used by “Other Usage” you can find this by using the following command.
find / -user $USER > /root/list_of_files.txt
Change $USER with the username of the account you are trying to find the files for. This will search the entire server for all files owned by that username, and dump them into a file in /root called list_of_files.txt