Atoms are identifiers and are therefore unique. However, an atom and a resource identifier can coincide. The string associated with an atom is called the "atom name". The name of an atom cannot be changed after creation, and no two atoms can have the same name. As a result, the name of an atom is commonly used to indicate the atom: “the atom codice_48” means, more precisely, “the atom whose associated string is codice_48.” or “the atom whose name is codice_48.” A client can request the creation of a new atom and can request for the atom (the identifier) of a given string. Some atoms are "predefined" (created by the server with given identifier and string).