What is the "\" character used for in PHP at the beginning of writing when calling functions?

Question

PHP

This symbol is not in the documentation, but it is used in some codes. For what?

$file = new \SplFileObject(\DIR_SYSTEM . $pathToController);

Answer 1

Well, how is it "not in the documentation" when a whole section is devoted to this? https://www.php.net/manual/en/language.namespaces.php