Call numbers are the combinations of letters and numbers that you find on the spine of a book. Each call number provides a unique "address" to help you locate items on the shelf.
SPH Libraries use the Dewey Decimal Classification system which uses 10 main classes to identify areas of knowledge. Each main class is further structured into ten hierarchical divisions, each having ten sections of increasing specificity.
[image courtesy of Maggie Appleton from http://www.thelibrarianedge.com/libedge/2016/8/29/dewey-designs-to-share]