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I'll take a stab: they are queues, indicating annotation and metadata changes have not been written to the metadata.opf files in the book folders. The updating of the metadata.opf files in the book folders is done in a separate low priority thread - the dirtied tables are neccessited by the need to catch up if calibre is terminated before the queues are emptied.
I am not 100% sure annotation backups are stored in the metadata.opf file, but…
BR
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