Hi @gymusic, as cPanel runs on Linux servers, it would follow the file naming system for that operating system. You can read about specific rules for Linux files here: https://www.cyberciti.biz/faq/linuxunix-rules-for-naming-file-and-directory-names/.
MPC, TNX for your reply and the link. It appears from your link, that underscore CAN be used in a file name, yet is not working for me. To be more specific... I have a sub menu "/pix/" under the public_html menu. They are JPG image files. When I place file names in a browser, the ones with a _ in the file name, I get a "file not found 404". All other files can be seen.