When the books are shelved by their classification system number then books on similar subjects are located together on the shelves. This is called collocation in library terminology. Collocation has an important function for organizing a library and allows for browsing of the shelves or stacks. Browsing is an important technique to learn in any library. It is easy to see what a library has by browsing through the shelves around the call number of your subject area.
Browsers can sometimes find much more than they are looking for just by looking at books nearby.
Dewey call numbers are entirely numeric. LC call numbers start with a one or two letter code followed by a number and then decimal. The following page has a breakdown of the LC call number schedule for education. The creators of subject heading lists try to categorize all of human knowledge into a logical order. When an item is cataloged in a library collection the cataloger uses this list to determine what subject headings best fit the content of the item.