Many others are considered but rejected because of selection critieria. This whole process is very much like what a library does when it selects, acquires, catalogs, and weeds books. When you later search the tree, it is very much like finding a library book by walking down the book stacks, following the classification system until you reach the appropriate call number. Some subject trees specialize in narrow subject fields, while others are general directories that attempt to encompass the entire universe of knowledge. This might be likened to comparing a university library with a special library.
Some subject trees operate entirely as directories of directories.
In other words, they point only to other directories, and not to the end pages themselves. Directories are not all based on the same kind of heirarchy. Some trees group their entries by a form of Dewey Decimal System, some by a variation of the Library of Congress System, while most use their own proprietary subject thesaurii. This is, at the very least, a quick way to build up your vocabulary in the field so you can do an effective keyword search. Whereas the reports from a search engine are totally unevaluated, human beings have evaluated each page listed in a subject directory.