Not entirely sure i get what your trying to accomplish.
A relational table store IDs from 2 tables to create a many to many relationship.
A hidden element is used to store the parent ID,
the checkbox stores the child table ID
in the case of the manage relational table tutorial it is relating Items to genres.
The hidden element stores the ItemID, the checkboxes store the Genre ID.
When a checkbox is checked, it created a new record in the itemgenres table with the ItemID and the selected genre id.
you could add additional fields to the item genre table and use a text field to for that column, but if you where to replace the checkbox with a text field, you would need to input the genre ID manually.