Hi,
A string with format "PLUGIN_CMD_123456" is meant to be used with MENURESOURCE_COMMAND, not RegisterCommandPlugin().
It tells Cinema the command ID and name for the menu item to be added.
Note the sub-IDs returned from GetSubContainer() are specific to a command and aren't global to Cinema.
Maybe CommandData isn't what you really need. Scripts from the user folder are automatically loaded and can be added to any menu using Customize Menus dialog.