Retrieval of materials is usually by special request. Items in closed stacks cannot be physically browsed, although they can also not be disrupted or stolen by library users; closed stacks often house a libraries' most valuable materials. They may also hold items that are fragile, or unusable.
Closed stacks are not limited to just sections of libraries. Some whole libraries are closed stacks in that they are not open to public view but limited to professional library staff or other specially trained individuals.