For Epub books the data comes from the "<dc:subject>some tag name</dc:subject>" entries in the EPUB opf file.
I've been experimenting with this recently on my Inkpad 3 by:
1. Setting up "Genres" in Calibre as per
https://manual.calibre-ebook.com/sub_groups.html
2. Adding my books to subgroups under 1. above eg. Fantasy, Science Fiction, Crime etc
3. In calibre adding a metadata plugboard with:
Format: epub,
Device: Pocketbook740 (for the Inkpad 3),
Source template: {#genre} - assuming you called the custom column lookup name from 1. above #genre,
Destination field: tags
The metadata plugboard then replaces the existing tag entries for the book with the entries from the genre sub groups in the epub file saved to the pocketbook.
The pocketbook library app then imports the book using these tag entries.