There are a lot of commands that are available in a command window to give functionality over and above that exposed in the GUI, calibredb being one of them. Calibredb is covered
here in the User Manual for calibre.
I think it would be a good idea if the GUI had a link to call up the User Manual (probably means adding a Help topic to the main menu buttons) as it would make it a bit more obvious and accessible. Possibly even specific links to the main sections of the User Manual.