@
martin_w - my guess is that the reason they're shown in the catalogue is because some calibre CLI utility programs require them as input eg
calibredb set_metadata [options] id [/path/to/metadata.opf]
See
calibre - Command Line Interface
If you don't want them shown in the catalogue then you can customise the catalogue see
Creating Catalogs: Start here
To see the
id for an individual book select the book in the list and press 'i', or double click the cover in the book details. You can also display the
id in the Book Details pane by ticking its box in
Preferences->Look and Feel->Book details.
BR