My initial thought was that it referenced another taxonomy (i.e. an FK from wp_term_taxonomy.parent to wp_term_taxonomy.term_taxonomy_id). ORMs like Corcel are built under this assumption. However, I’ve done some digging and most information seems to indicate that it actually references wp_terms.term_id. In the case of Corcel, I think that this coincidentally works – WordPress will set the term_id and the term_taxonomy_id to the same value. If you’ve messed with the data, or applied your own imports / migrations, you might not get this same behaviour.