It is a pattern of book binding: the cover of the letter is often made of cardboard and blue cloth (also made of brocade), according to the size and thickness of the book. There are two forms of function sets. One is wrapped all around, exposing the upper and lower mouths of the book, which is called semi-wrapped; The other is to wrap all six sides of the book, which is called full package. In addition to the thick paper cover, there are two kinds of outer packaging: plywood and wooden cases. The splint type is to use two pieces of wood the same size as the book, clip them on the top and bottom of the book, and then tie them tightly with a cloth belt. A transaction box is made according to the size of a book, and the book is put in it.
Simply put, it is a layer of packaging outside the book cover.
There is a difference between a transaction box and a letter sleeve. As for whether it is possible to make a letter sleeve in the affairs box, I think it is not impossible, but it is not common.