Database – What is referencing wp_term_taxonomy.parent?

My first thought was that it was referring to another taxonomy (ie H. A FK) wp_term_taxonomy.parent to 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 wp_terms.term_id,

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.