Sometimes, the same book may have different call numbers depending on which library it is located, and depending on how the staff at that library wished to catalog the book. The call numbers are found on each book in a small white label, usually located at the base of the spine. In the catalog, these may be referred to as "shelf location."
Sometimes you may find the same book classified as YA, or young adult, literature at one location and J, or juvenile, at another. This will change where the book is found in the library, and you should always go by the call number information when looking for an item on the shelf.
In the photo example, we see the book Small Sacrifices by Ann Rule is classified as a biography at one location, resulting in a call number starting with the designation "B." A biography is a true story about a person or people. All biographies are classified by their subject matter, so we can tell by the DOW that comes next that the biography is about someone who's last name starts with those letters. We may not know who the book is about, but a little online research tells us the book is about a woman named Diane Downs. Someone already familiar with the case by her name may find the book more easily through this classification.
At another location, the same book is classified according to its subject matter using the Dewey Decimal system. The Dewey general category 364 means the book is about criminology. In this case, the call number includes the first three letters of the author's last name, RUL.
Because Dewey classifies books by subject matter, we can know that all books with the classification 364.1523 will be about murders. Books in this area of the non-fiction section of the library may also be referred to by their genre name, "true crime" novels.