My first thought was that it was referring to another taxonomy (ie H. A FK)
wp_term_taxonomy.term_taxonomy_id). ORMs like Corcel are built under this assumption. However, I dug a bit and most of the information seems to indicate that it is actually being referred to
In the case of Corcel, I think that happens by accident – WordPress will stop that
term_id and the
term_taxonomy_id to the same value. If you've studied the data or applied your own imports / migrations, you may not see the same behavior.