Books are classified and stored with index cards. Initially, there was a room in the library dedicated to cards with book information. These cards are classified according to the book number, and they contain the information of the title and main content. To borrow books, you must consult these cards first. When you find the book you need, fill in the book number on the borrowing list and give it to the librarian. Later, when the library opened, we could go directly to the stacks to find books. There are classified information of books on the shelf. Knowing these classified information will soon help you find the book you want.
For example, I know that there is a book whose classification number is B948-94. I will first look for the bookshelf classified by B, then find the location of 948, and then find 94, and I will find it.
Banks are not familiar with it, but if the bank's deposit records are classified by date, they will be easy to find.