I believe that Max used the underscore prefix, for custom ui.
Alphabetically, files with it occur in a list, before other files.
All underscores could be removed, so long as the new name, without underscore is used, and no name duplicate is created. Personally I have used underscore, just to separate "NONSTOCK" commands.
-Brian
Nowadays, I keep most nonstock commands in a MoI Appdata command subfolder, with the nonessential underscore prefix. They stay there during MoI upgrades and betas.