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It is worth noting that as of the latest release (or thereabouts), composite custom column templates are saved in the metadata_db_prefs_backup.json file every time you exit the calibre gui. This file is used by restore to rebuild the custom columns, as well as to restore a lot of other things like saved searches, user categories, plugboards, grouped search terms, custom template functions, and several library-level preferences.
The custom column descriptions in the metadata.opf files are used only if the metadata_db_prefs_backup.json is missing or corrupted.
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