Has parent field in the table wp_term_taxonomy has term_id or term_taxonomy_id

I have created a category “Book” with a sub category “comics”,
to check how wordpress is maintaining the hierarchy structure.
When i check the table “wp_term_taxonomy”, I found that such term with a parent has “parent” field populated . As the term id and term_taxonomy_id have the same value for the parent “term name” I am unable to crack whether
in the table wp_term_taxonomy do the field, has term_taxonomy_id of the parent term or term_id of the parent