A sewn bound finish is characterized by the signatures being joined together with thread. This is the strongest way to finish a book.
When yarnless broached the hangovers are not bound with thread. Just before glueing the hangovers, a small part of the back is milled.
PUR is cooked according to the same steps as yarnless broched. The only difference is the glue. PUR glue offers more possibilities than yarnless